feat(cache): use repository-local mirror caches

[realityforge]

Summary

• default Braid caching to repository-local per-mirror bare caches under .git/braid/cache
• keep explicit --global-cache-dir / BRAID_GLOBAL_CACHE_DIR as shared full mirror caches and reject old --cache-dir / BRAID_LOCAL_CACHE_DIR controls
• hydrate setup caches under a per-cache lock, fetch full SHAs shallow when possible, and fall back to full per-mirror fetches when needed
• update docs, completion, and integration tests for the new cache model

Validation

• bazel run @rules_go//go -- fmt ./...
• git --version
• git merge-tree --write-tree "--merge-base=HEAD^{tree}" "HEAD^{tree}" "HEAD^{tree}"
• bazel test --test_env=PATH //...
• bazel run @rules_go//go -- vet ./...
• bazel run @rules_go//go -- run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.0 run
• bazel test --test_env=PATH //integration/...
• git diff --exit-code