refactor(serve)!: remove JGit engine, make subprocess the only git backend

[tinder-maxwellelliott]

What & why

Makes the native git subprocess (ProcessGitClient) the only git engine for bazel-diff serve and removes the in-process JGit integration entirely, per request to "make subprocess the only option for now."

This also resolves the pending partial-clone checkout failure JGit 5.13 could not handle — it has no promisor/lazy-fetch, so checking out a revision whose trees/blobs weren't materialized by the initial clone threw JGit checkout … Missing unknown <sha>. Native git fetches those blobs on demand at checkout, so the previously-proposed JGitClient.checkout() fallback is now moot.

Changes

• Deleted JGitClient.kt, JGitClientTest.kt, and Slf4jBindingTest.kt (the slf4j-nop runtime dep existed only to silence JGit's NOP-logger warning).
• Dropped the org.eclipse.jgit, slf4j-nop, and slf4j-api Maven deps from MODULE.bazel / cli/BUILD and repinned maven_install.json (−111 lines: jgit, JavaEWAH, both slf4j jars).
• Removed the --gitEngine flag; ServeCommand.createGitClient() now always returns ProcessGitClient. Updated the --gitPath help text (no longer conditioned on the removed flag).
• /metrics gitEngine now reports the literal "subprocess". The field is intentionally kept so the metrics JSON contract stays stable (and is easy to re-expand if another engine returns).
• serve_harness.py: removed the engine dimension — the harness existed to assert cross-engine parity, which no longer applies. Cases are now full / full+deps / shallow / partial.
• Docs: README/template prose, the /metrics example, and the regenerated serve --help block.

Validation

• //cli:bazel-diff builds clean without JGit.
• Serve/server unit tests pass: ServeCommandTest, MetricsServiceTest, BazelDiffServerTest, GitClientTest, HashServiceTest, ImpactedTargetsServiceTest, CachePrunerTest, LocalDiskHashCacheStorageTest, StderrLoggerTest.
• E2ETest#testServeEndToEnd passes — it stands up a real git repo and runs serve with no --gitEngine, so it now exercises the subprocess (ProcessGitClient) path end-to-end (resolve/checkout + real bazel query + distances + cache).
• ktfmt and buildifier clean.
• The 4 unrelated E2ETest failures (cquery/bzlmod/external-repo/Android) are the pre-existing sandbox environment issue (nested bazel can't reach the network / locate a JDK for coursier), not caused by this change.

⚠️ Breaking change

• The --gitEngine flag is removed. Invocations passing --gitEngine=subprocess should drop the flag; --gitEngine=jgit (the previous default) is no longer available.
• A git binary must now be on PATH (the old default JGit engine required none).

Suggest a major version bump (→ 33.0.0) on release. If softer back-compat is preferred, --gitEngine could instead be kept as a hidden, deprecated no-op that accepts only subprocess — happy to add that.

🤖 Generated with Claude Code