Improve hook handler quality: fix bugs, simplify, add tests

[svarlamov]

Fix bugs, simplify parsing, add unit tests for hook handlers

Summary

Follow-up quality improvements to the hook handlers introduced in PR #530. All changes in src/commands/git_hook_handlers.rs:

Bug fixes:

• Non-managed hooks silently broken in hooks-only mode (Round 3): When git-ai takes over core.hooksPath for a repo, only MANAGED_GIT_HOOK_NAMES were installed as symlinks. Non-managed core hooks (e.g. commit-msg, pre-merge-commit) had no symlinks in the managed hooks directory, so pre-existing repo-level or global hooks for those events silently stopped executing. Now selectively installs symlinks (to the git-ai binary) for non-managed hooks only when the corresponding script actually exists in the forward target directory. This avoids unnecessary hook interception while preserving $0/dirname for Husky-style hooks that resolve relative paths. Stale symlinks are cleaned up when the original hook is removed.
• handle_git_hook_invocation called should_forward_repo_state_first(None) twice per hook invocation (once to check existence, once inside execute_forwarded_hook). Now cached into cached_forward_dir and threaded through.
• latest_cherry_pick_source_from_sequencer used is_valid_git_oid (requires exactly 40 or 64 hex chars) to validate entries in the sequencer done file, which can contain abbreviated SHAs. Added is_valid_git_oid_or_abbrev (≥7 hex chars) and use it there instead.
• maybe_enable_rebase_hook_mask called SystemTime::now() twice — once for session_id and once for created_at_unix_ms — producing potentially inconsistent timestamps. Now computes once and reuses.
• handle_rebase_post_rewrite_from_stdin: .last().unwrap_or_else(|| original_commits[0].clone()) had a dead-code panic fallback (vec is guaranteed non-empty by earlier is_empty() guard). Simplified to .last().unwrap().

Simplifications:

• Extracted sync_non_managed_hook_symlinks to scan the forward target directory and selectively install/clean up non-managed hook symlinks. Extracted remove_hook_entry helper to deduplicate cleanup logic.
• Extracted parse_whitespace_fields(stdin, min_fields) to consolidate the duplicated split_whitespace + filter logic between parse_hook_stdin and parse_reference_transaction_stdin.
• Removed 20-line explicit no-op match arm in run_managed_hook ("commit-msg" | "pre-merge-commit" | ...) that was already covered by the _ => 0 wildcard.
• Extracted is_null_oid helper to replace 5 scattered chars().all(|c| c == '0') patterns.
• Rewrote hook_has_no_managed_behavior to derive from MANAGED_GIT_HOOK_NAMES constant instead of manually maintaining a 20-entry match list. Self-maintaining and can't drift.
• Shared git_dir lookup in hook_requires_managed_repo_lookup reference-transaction arm (was calling git_dir_from_context() twice via separate helper functions).
• Removed dead is_cherry_pick_in_progress_from_context function (inlined into single call site).

Tests (31 total, +26 new):
Unit tests for is_valid_git_oid, is_valid_git_oid_or_abbrev, is_null_oid, parse_hook_stdin, parse_reference_transaction_stdin, parse_whitespace_fields, is_path_inside_component, is_path_inside_any_git_ai_dir, hook_has_no_managed_behavior (3 tests), CherryPickBatchState serde roundtrip, RepoHookState serde roundtrip, ForwardMode::None serialization, ensure_hook_symlink idempotency, rebase hook mask double-enable safety, hook name subset invariants, and non-managed hook forwarding behavior (4 tests: provisioned-only-when-original-exists, resync cleanup, no-forward-target skips non-managed, forward-target with empty dir).

Review & Testing Checklist for Human

• Verify selective non-managed hook forwarding (CRITICAL): Non-managed hooks (commit-msg, pre-merge-commit, etc.) now get symlinks to the git-ai binary only when the corresponding script exists in the forward target directory (e.g., .husky/commit-msg). Test with Husky/lefthook: (1) verify hooks execute correctly, (2) verify $0 and dirname "$0" resolve to the original hook path (not .git/ai/hooks), (3) add a new hook to the forward dir and run git-ai git-hooks ensure to verify it gets picked up, (4) remove a hook from the forward dir and verify the symlink is cleaned up on next ensure.
• Verify self-healing doesn't miss hook changes: Self-healing only triggers when core.hooksPath changes, not when individual hook files are added/removed in the forward dir. If a user adds a new hook to their hooks dir, it won't be picked up until the next git-ai git-hooks ensure or a self-heal trigger. Verify this is acceptable behavior or if we need to add file-watching.
• Test non-managed hook execution path: Verify the full flow: git invokes symlink → git-ai binary → handle_git_hook_invocation → execute_forwarded_hook (forwards to original) → run_managed_hook (no-op for non-managed). Confirm forwarding happens before managed logic and that non-managed hooks execute correctly.
• Confirm is_valid_git_oid_or_abbrev threshold: Verify that 7 hex chars is sufficient for git's abbreviated OID format. Git's default core.abbrev is "auto" which can go as low as 4 in small repos. Check if sequencer done files ever contain <7 char abbreviations.
• Run integration tests: Only unit tests were run locally (cargo test --lib). Run full test suite including tests/hook_modes.rs and tests/stash_attribution.rs to verify no regressions. Run the Heavy Matrix and 10x Nasty Rebase benchmarks from PR Add dual-mode git-ai operation via global hooks and wrapper #530 to confirm no performance regression.

Notes

• All 31 unit tests pass locally (18 from round 1, 5 from round 2, 8 from round 3)
• cargo check and cargo clippy pass with no warnings
• Link to Devin run: https://app.devin.ai/sessions/54015db92a4e43dc8c7c08d57ad7e520
• Requested by: @svarlamov

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