fix(extraction): keep git-aware scans working before Git 2.36 (#1793)

Land upstream PR #1604 by @maxmilian from commit
27c149524d968485164fb43bdec994fdb9323c75 on current main cece0720.
Fixes #1549.

Use the upstream lsFilesStaged helper at both collection and embedded-repo
discovery call sites. Retry without --recurse-submodules when staged
recursive listing fails, retaining the mode bits needed for gitlinks.

Keep the upstream regression test byte-for-byte and preserve main's
existing extraction/watcher changes, including #1728. Resolve the
changelog conflict with a concise Unreleased entry, and keep the existing
collectGitFiles documentation attached to its function.

Verified on Linux x86_64 with Node 22.19.0 and Git 2.47.3:
- The unchanged upstream test fails against main: expected [ 'a.ts' ]
  to include 'dir_b/b.ts'; it passes with the fix.
- The same persistent fixture under a PATH shim rejecting -s with
  --recurse-submodules (exit 128) changes scanDirectory from [a.ts]
  to [a.ts, dir_b/b.ts], and gitlink watcher roots from [] to [lib/].
- Real-Git controls return both files and the watcher root in both arms.
- 69 related scope/config tests and 23 extraction scanning tests pass.
- npx tsc --noEmit passes.

Co-authored-by: Colby McHenry <colbymchenry@users.noreply.github.com>
Co-authored-by: Max Hsu <maxmilian@gmail.com>
Co-authored-by: newshowardz777 <newshowardz777@users.noreply.github.com>

CHANGELOG.md

@@ -151,6 +151,8 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 - `codegraph install` now honors `CLAUDE_CONFIG_DIR` and `CODEX_HOME` for global Claude Code and Codex setup so CodeGraph loads in your chosen profile (thanks @seanchann; #1627).
 
+- Files opted in with `includeIgnored` now stay indexed on Git older than 2.36, and embedded repositories remain visible to the watcher (thanks @maxmilian and @newshowardz777; #1549).
+
 #### Screens, links and navigation
 
 - **Where the app goes after login is a fork, not two always-es.** A navigation whose destination comes back from a helper — `router.replace(await resolvePostLoginRoute())` over `return (await hasSeenWelcome(…)) ? '/home/' : '/welcome/'` — drew both screens with no condition, reading as if the welcome screen always shows. The two arms share a line, and only a column can tell them apart; each synthesized edge now carries its literal's own position, so the guard reader says which arm it is: `WHEN await hasSeenWelcome(…)` → home, and its negation → welcome. And the scan starts at the helper's body, so a literal-union return type — `Promise<'/welcome/' | '/home/'>`, whose routes are string literals too, written first — no longer stands in for the navigation itself. Re-index after upgrading to pick the positions up.