feat(cli): implement 'lore trace' command (bd-2n4, bd-9dd)

Gate 5 Code Trace - Tier 1 (API-only, no git blame).
Answers 'Why was this code introduced?' by building
file -> MR -> issue -> discussion chains.

New files:
- src/core/trace.rs: run_trace() query logic with rename-aware
  path resolution, entity_reference-based issue linking, and
  DiffNote discussion extraction
- src/core/trace_tests.rs: 7 unit tests for query logic
- src/cli/commands/trace.rs: CLI command with human output,
  robot JSON output, and :line suffix parsing (5 tests)

Human output shows full content (no truncation).
Robot JSON truncates discussion bodies to 500 chars for token efficiency.

Wiring:
- TraceArgs + Commands::Trace in cli/mod.rs
- handle_trace in main.rs
- VALID_COMMANDS + robot-docs manifest entry
- COMMAND_FLAGS autocorrect registry entry

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main.rs

@@ -11,26 +11,26 @@ use lore::cli::autocorrect::{self, CorrectionResult};
 use lore::cli::commands::{
     IngestDisplay, InitInputs, InitOptions, InitResult, ListFilters, MrListFilters,
     NoteListFilters, SearchCliFilters, SyncOptions, TimelineParams, open_issue_in_browser,
-    open_mr_in_browser, print_count, print_count_json, print_doctor_results, print_drift_human,
-    print_drift_json, print_dry_run_preview, print_dry_run_preview_json, print_embed,
-    print_embed_json, print_event_count, print_event_count_json, print_file_history,
+    open_mr_in_browser, parse_trace_path, print_count, print_count_json, print_doctor_results,
+    print_drift_human, print_drift_json, print_dry_run_preview, print_dry_run_preview_json,
+    print_embed, print_embed_json, print_event_count, print_event_count_json, print_file_history,
     print_file_history_json, print_generate_docs, print_generate_docs_json, print_ingest_summary,
     print_ingest_summary_json, print_list_issues, print_list_issues_json, print_list_mrs,
     print_list_mrs_json, print_list_notes, print_list_notes_csv, print_list_notes_json,
     print_list_notes_jsonl, print_search_results, print_search_results_json, print_show_issue,
     print_show_issue_json, print_show_mr, print_show_mr_json, print_stats, print_stats_json,
     print_sync, print_sync_json, print_sync_status, print_sync_status_json, print_timeline,
-    print_timeline_json_with_meta, print_who_human, print_who_json, query_notes, run_auth_test,
-    run_count, run_count_events, run_doctor, run_drift, run_embed, run_file_history,
-    run_generate_docs, run_ingest, run_ingest_dry_run, run_init, run_list_issues, run_list_mrs,
-    run_search, run_show_issue, run_show_mr, run_stats, run_sync, run_sync_status, run_timeline,
-    run_who,
+    print_timeline_json_with_meta, print_trace, print_trace_json, print_who_human, print_who_json,
+    query_notes, run_auth_test, run_count, run_count_events, run_doctor, run_drift, run_embed,
+    run_file_history, run_generate_docs, run_ingest, run_ingest_dry_run, run_init, run_list_issues,
+    run_list_mrs, run_search, run_show_issue, run_show_mr, run_stats, run_sync, run_sync_status,
+    run_timeline, run_who,
 };
 use lore::cli::render::{ColorMode, GlyphMode, Icons, LoreRenderer, Theme};
 use lore::cli::robot::{RobotMeta, strip_schemas};
 use lore::cli::{
     Cli, Commands, CountArgs, EmbedArgs, FileHistoryArgs, GenerateDocsArgs, IngestArgs, IssuesArgs,
-    MrsArgs, NotesArgs, SearchArgs, StatsArgs, SyncArgs, TimelineArgs, WhoArgs,
+    MrsArgs, NotesArgs, SearchArgs, StatsArgs, SyncArgs, TimelineArgs, TraceArgs, WhoArgs,
 };
 use lore::core::db::{
     LATEST_SCHEMA_VERSION, create_connection, get_schema_version, run_migrations,
@@ -40,8 +40,10 @@ use lore::core::error::{LoreError, RobotErrorOutput};
 use lore::core::logging;
 use lore::core::metrics::MetricsLayer;
 use lore::core::paths::{get_config_path, get_db_path, get_log_dir};
+use lore::core::project::resolve_project;
 use lore::core::shutdown::ShutdownSignal;
 use lore::core::sync_run::SyncRunRecorder;
+use lore::core::trace::run_trace;
 
 #[tokio::main]
 async fn main() {
@@ -199,6 +201,7 @@ async fn main() {
         Some(Commands::FileHistory(args)) => {
             handle_file_history(cli.config.as_deref(), args, robot_mode)
         }
+        Some(Commands::Trace(args)) => handle_trace(cli.config.as_deref(), args, robot_mode),
         Some(Commands::Drift {
             entity_type,
             iid,
@@ -725,6 +728,7 @@ fn suggest_similar_command(invalid: &str) -> String {
         ("note", "notes"),
         ("drift", "drift"),
         ("file-history", "file-history"),
+        ("trace", "trace"),
     ];
 
     let invalid_lower = invalid.to_lowercase();
@@ -766,6 +770,7 @@ fn command_example(cmd: &str) -> &'static str {
         "generate-docs" => "lore --robot generate-docs",
         "embed" => "lore --robot embed",
         "robot-docs" => "lore robot-docs",
+        "trace" => "lore --robot trace src/main.rs",
         "init" => "lore init",
         _ => "lore --robot <command>",
     }
@@ -1888,6 +1893,51 @@ fn handle_file_history(
     Ok(())
 }
 
+fn handle_trace(
+    config_override: Option<&str>,
+    args: TraceArgs,
+    robot_mode: bool,
+) -> Result<(), Box<dyn std::error::Error>> {
+    let start = std::time::Instant::now();
+    let config = Config::load(config_override)?;
+
+    let (path, line_requested) = parse_trace_path(&args.path);
+
+    if line_requested.is_some() && !robot_mode {
+        eprintln!(
+            "Note: Line-level tracing requires Tier 2 (git blame). Showing file-level results."
+        );
+    }
+
+    let project = config
+        .effective_project(args.project.as_deref())
+        .map(String::from);
+
+    let db_path = get_db_path(config.storage.db_path.as_deref());
+    let conn = create_connection(&db_path)?;
+    let project_id = project
+        .as_deref()
+        .map(|p| resolve_project(&conn, p))
+        .transpose()?;
+
+    let result = run_trace(
+        &conn,
+        project_id,
+        &path,
+        !args.no_follow_renames,
+        args.discussions,
+        args.limit,
+    )?;
+
+    if robot_mode {
+        let elapsed_ms = start.elapsed().as_millis() as u64;
+        print_trace_json(&result, elapsed_ms, line_requested);
+    } else {
+        print_trace(&result);
+    }
+    Ok(())
+}
+
 async fn handle_timeline(
     config_override: Option<&str>,
     args: TimelineArgs,
@@ -2556,6 +2606,16 @@ fn handle_robot_docs(robot_mode: bool, brief: bool) -> Result<(), Box<dyn std::e
                 "active_minimal": ["entity_type", "iid", "title", "participants"]
             }
         },
+        "trace": {
+            "description": "Trace why code was introduced: file -> MR -> issue -> discussion. Follows rename chains by default.",
+            "flags": ["<path>", "-p/--project <path>", "--discussions", "--no-follow-renames", "-n/--limit <N>"],
+            "example": "lore --robot trace src/main.rs -p group/repo",
+            "response_schema": {
+                "ok": "bool",
+                "data": {"path": "string", "resolved_paths": "[string]", "trace_chains": "[{mr_iid:int, mr_title:string, mr_state:string, mr_author:string, change_type:string, merged_at_iso:string?, updated_at_iso:string, web_url:string?, issues:[{iid:int, title:string, state:string, reference_type:string, web_url:string?}], discussions:[{discussion_id:string, mr_iid:int, author_username:string, body_snippet:string, path:string, created_at_iso:string}]}]"},
+                "meta": {"tier": "string (api_only)", "line_requested": "int?", "elapsed_ms": "int", "total_chains": "int", "renames_followed": "bool"}
+            }
+        },
         "file-history": {
             "description": "Show MRs that touched a file, with rename chain resolution and optional DiffNote discussions",
             "flags": ["<path>", "-p/--project <path>", "--discussions", "--no-follow-renames", "--merged", "-n/--limit <N>"],