refactor: extract unit tests into separate _tests.rs files

Move inline #[cfg(test)] mod tests { ... } blocks from 22 source files
into dedicated _tests.rs companion files, wired via:

    #[cfg(test)]
    #[path = "module_tests.rs"]
    mod tests;

This keeps implementation-focused source files leaner and more scannable
while preserving full access to private items through `use super::*;`.

Modules extracted:
  core:      db, note_parser, payloads, project, references, sync_run,
             timeline_collect, timeline_expand, timeline_seed
  cli:       list (55 tests), who (75 tests)
  documents: extractor (43 tests), regenerator
  embedding: change_detector, chunking
  gitlab:    graphql (wiremock async tests), transformers/issue
  ingestion: dirty_tracker, discussions, issues, mr_diffs

Also adds conflicts_with("explain_score") to the --detail flag in the
who command to prevent mutually exclusive flags from being combined.

All 629 unit tests pass. No behavior changes.

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

src/cli/mod.rs

@@ -905,7 +905,12 @@ pub struct WhoArgs {
     pub fields: Option<Vec<String>>,
 
     /// Show per-MR detail breakdown (expert mode only)
-    #[arg(long, help_heading = "Output", overrides_with = "no_detail")]
+    #[arg(
+        long,
+        help_heading = "Output",
+        overrides_with = "no_detail",
+        conflicts_with = "explain_score"
+    )]
     pub detail: bool,
 
     #[arg(long = "no-detail", hide = true, overrides_with = "detail")]