Add native history and harden Git workflows

[hbmartin]

Summary

• add native history, commit diff, file preview, and working-tree views with dark-mode-aware rendering
• make repository loading, selection, diff generation, task execution, and auto-fetch behavior asynchronous, cancellable, and race-safe
• harden patch generation and staging for renames, binary files, empty repositories, submodules, and partial selections
• expand preferences, coverage, unit/integration/UI tests, sanitizer plans, formatting checks, static analysis, and CI verification
• include the complete current worktree, ignore local reviews_triage/ output, and pin the ObjectiveGit reliability update

Why

Several native workflows could block the main thread, render stale selections after rapid navigation, mishandle edge-case paths or partial patches, and leave auto-fetch permanently paused after transient failures. The existing CI suite also did not exercise enough of these paths or enforce formatting, coverage, and analyzer regressions.

This change moves expensive Git/rendering work off the UI thread, adds cancellation and generation guards, bounds subprocesses and fetch retry behavior, strengthens Git edge-case handling, and adds regression coverage around the affected flows.

Dependency

Depends on gitx/objective-git#97, which makes the libgit2 update script deterministic and validates the built architecture before the parent submodule pointer advances.

Validation

• scripts/verify_static.sh origin/master
• 34 unit/integration tests passed; 0 failed and 0 skipped
• all per-target coverage floors passed (GitX app coverage: 16.84%, floor: 16.50%)
• Clang static analysis succeeded; the warning-budget check passed at the measured 16-warning legacy baseline
• sh -n External/objective-git/script/update_libgit2

[hannesa2]

The AI changed here too much without knowing why it's there 😁