feat(config): add AllTargetsFiles to invalidate all targets on trigger file change

[yushan8]

Summary

• Adds RepositoryConfig.AllTargetsFiles: a list of repo-relative file paths that, when their content hash differs between two revisions, cause GetChangedTargets to report every target in the second revision as changed (distance 0)
• File hashes are computed from git ls-tree during graph computation in targethasher.FromProto, stored as sidecar data in Metadata.AllTargetsFileHashes, and compared at diff time in the controller
• Intended for files that affect the build globally but are invisible to the target graph (toolchain definitions, CI config, Bazel wrapper scripts)
• Gob-format graphs carry this field automatically via gob serialization

Data flow

git ls-tree → HashConfig.KnownSourceHashes
  → targethasher.FromProto filters by AllTargetsFiles
  → Result.AllTargetsFileHashes
  → mapper → entity.Metadata.AllTargetsFileHashes
  → stored in graph blob (gob auto-serialization)
  → read back at comparison time in controller
  → if any hash differs → all targets reported as changed

Example config

repository:
  - remote: "git@github.com:org/repo.git"
    all_targets_files:
      - .bazelrc
      - .bazelversion
      - tools/bazel
      - rules/go/sdk.bzl

Test plan

• allTargetsFileChanged unit tests — nil/empty/matching/differing hash scenarios (table-driven)
• compareTargetGraphs gob-path test — trigger fires → all targets changed
• compareTargetGraphs gob-path test — matching hashes → normal diff
• Mapper propagation tests — Result → Metadata, including via ResultToGraphChunks
• All existing tests pass

Stack: TGB format support in PR #285