test sweep: cover src/git/client.dart (T-91)

23 integration tests in test/git/client_test.dart driving a real git
binary against per-test sandbox repos. Covers every GitClient public
method that wasn't already exercised through the daemon command
suite:

- Queries: status (clean + with upstream tracking ahead/behind),
  diff with explicit paths, log, currentBranch, branches (with the
  current-marker check), repoRoot (in-repo + non-repo paths).
- Mutations: unstage with paths, commit-returns-hash, discard,
  discard with empty list, stash + stashPop with includeUntracked,
  checkout, stageHunk + unstageHunk via _applyPatch.
- Error surface: mutations throw GitException on non-zero exit,
  _applyPatch surfaces stderr from a bad patch, bad git binary path
  → ProcessException → GitException, queries return empty fallbacks
  on a non-git workDir.
- parseLog standalone parser: empty input, short-record skip, full
  record with body, 5-field record without body.

Coverage: src/git/client.dart 76/141 -> 124/141 (88%). The remaining
17 lines are the pull / push paths (would need a real remote with
matching credentials — out of scope) and the status GitException
catch branches that require an in-flight git process failure mid-call.

Total coverage 75.42% -> 76.04%; floor bumped to 76.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pubspec.yaml

@@ -18,7 +18,7 @@ repository: https://github.com/postmeridiem/clide
 
 # Pre-push line-coverage floor. Ratchets up only — see D-66.
 # Reading: `awk -F: '/^coverage_floor:/ {gsub(/ /,"",$2); print $2}' pubspec.yaml`.
-coverage_floor: 75
+coverage_floor: 76
 
 # Project metadata (was project.yaml, folded in per D-056).
 # version: above is the single source of truth. The Makefile reads