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/cli/mod.rs

@@ -238,6 +238,9 @@ pub enum Commands {
     #[command(name = "file-history")]
     FileHistory(FileHistoryArgs),
 
+    /// Trace why code was introduced: file -> MR -> issue -> discussion
+    Trace(TraceArgs),
+
     /// Detect discussion divergence from original intent
     Drift {
         /// Entity type (currently only "issues" supported)
@@ -1006,6 +1009,38 @@ pub struct FileHistoryArgs {
     pub limit: usize,
 }
 
+#[derive(Parser)]
+#[command(after_help = "\x1b[1mExamples:\x1b[0m
+  lore trace src/main.rs                      # Why was this file changed?
+  lore trace src/auth/ -p group/repo          # Scoped to project
+  lore trace src/foo.rs --discussions          # Include DiffNote context
+  lore trace src/bar.rs:42                    # Line hint (Tier 2 warning)")]
+pub struct TraceArgs {
+    /// File path to trace (supports :line suffix for future Tier 2)
+    pub path: String,
+
+    /// Scope to a specific project (fuzzy match)
+    #[arg(short = 'p', long, help_heading = "Filters")]
+    pub project: Option<String>,
+
+    /// Include DiffNote discussion snippets
+    #[arg(long, help_heading = "Output")]
+    pub discussions: bool,
+
+    /// Disable rename chain resolution
+    #[arg(long = "no-follow-renames", help_heading = "Filters")]
+    pub no_follow_renames: bool,
+
+    /// Maximum trace chains to display
+    #[arg(
+        short = 'n',
+        long = "limit",
+        default_value = "20",
+        help_heading = "Output"
+    )]
+    pub limit: usize,
+}
+
 #[derive(Parser)]
 pub struct CountArgs {
     /// Entity type to count (issues, mrs, discussions, notes, events)