Add clean-slate design doc and begin gitoxide migration of git wrapper

[0lut]

Summary

Commits the clean-slate read-through cache design write-up as docs/clean-slate-design.md and starts the highest-leverage bet from it: serving Git operations in-process via gitoxide (gix) instead of shelling out to the git binary.

This first slice migrates the five lowest-risk operations — all local, read-only, never touching the network:

• rev_parse → gix::Repository::rev_parse_single
• for_each_ref / for_each_ref_commits → references().prefixed(prefix) (symbolic refs followed without peeling annotated tags, matching %(objectname))
• is_ancestor → rev_walk([ancestor]).with_hidden([descendant]) is-empty check (mirrors rev-list --max-count=1 A --not B)
• cat_file_batch_types → try_find_header per id, missing objects skipped (mirrors cat-file --batch-check; gix never lazy-fetches, so this also covers the GIT_NO_LAZY_FETCH variant)

Wiring:

• New gix_backend module in git-cache-git with sync implementations run via spawn_blocking, bounded by the same semaphore as git subprocesses; repos opened with gix::open::Options::isolated() (no user/system config), mirroring the subprocess env_clear hygiene.
• Argument validation (reject_revision_arg, reject_ref_arg) still runs first even though there's no argv to inject into.
• Kill switch: AppConfig.use_gitoxide (env GIT_CACHE_USE_GITOXIDE, default true); Git::with_gitoxide(false) routes everything back through the git binary.
• gix 0.84 pinned with default-features = false, features = ["revision", "sha1"] — no network features compiled in.

Testing: new gitoxide_backend_matches_subprocess_backend parity test asserts identical results from both backends for every migrated operation (including missing-object and self-ancestor edge cases); the full existing suite now exercises the gix path by default since the flag defaults to on.

Network operations (fetch_*, ls_remote_*, upload_pack_spawn) intentionally stay on the subprocess path; they migrate in later PRs per the design doc.

Link to Devin session: https://app.devin.ai/sessions/a5e627993756406f84d6d0b40d3a4165
Requested by: @0lut

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[0lut]

we should have GixBackend and GitBackend, so the methods like rev_parse should be part of the interface definition and they are shared, instead of if/else should use paths like: https://github.com/0lut/gitmirrorcache/pull/75/changes#diff-33ad69c17787788f144c92e52eed95296ddecf90a61fdf0de42c368d908857f1R166-R171

[devin-ai-integration]

Done in 76b4dcd: added a LocalGitBackend trait with GixBackend and GitBackend implementations (new crates/git-cache-git/src/backend.rs). rev_parse, is_ancestor, for_each_ref/for_each_ref_commits, and cat_file_batch_types are now part of the shared interface; the Git wrapper just validates inputs and delegates to self.local_backend — no more if use_gitoxide branches. with_gitoxide(bool) now swaps the backend Arc so the existing config/env kill switch and parity test keep working.