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/commands/mod.rs

@@ -14,6 +14,7 @@ pub mod stats;
 pub mod sync;
 pub mod sync_status;
 pub mod timeline;
+pub mod trace;
 pub mod who;
 
 pub use auth_test::run_auth_test;
@@ -48,4 +49,5 @@ pub use stats::{print_stats, print_stats_json, run_stats};
 pub use sync::{SyncOptions, SyncResult, print_sync, print_sync_json, run_sync};
 pub use sync_status::{print_sync_status, print_sync_status_json, run_sync_status};
 pub use timeline::{TimelineParams, print_timeline, print_timeline_json_with_meta, run_timeline};
+pub use trace::{parse_trace_path, print_trace, print_trace_json};
 pub use who::{WhoRun, print_who_human, print_who_json, run_who};