Add commit_tree_hash to checkpoint metadata

[arosenfeld2003]

Summary

• Store the git tree hash of HEAD at condensation time in checkpoint metadata (commit_tree_hash field)
• This provides a content-addressable anchor that survives history rewrites (rebase, squash, force-push) — a rebased commit has a different SHA but the same tree hash when file content is unchanged
• A future entire repair command can use this field to re-link orphaned checkpoints by scanning the current branch for commits whose tree hash matches

Closes #321

Changes

• checkpoint/checkpoint.go — Add CommitTreeHash string field to WriteCommittedOptions, CommittedMetadata (json:"commit_tree_hash,omitempty"), and CheckpointSummary (json:"commit_tree_hash,omitempty")
• checkpoint/committed.go — Pass CommitTreeHash through in writeSessionToSubdirectory() and writeCheckpointSummary()
• strategy/common.go — Add GetHeadTreeHash() helper (same pattern as GetCurrentBranchName)
• strategy/manual_commit_condensation.go — Call helper and pass to WriteCommittedOptions
• strategy/auto_commit.go — Call helper and pass to options in both commitMetadataToMetadataBranch() and commitTaskMetadataToMetadataBranch()
• checkpoint/checkpoint_test.go — Round-trip test: write with tree hash and verify in both metadata levels; write without and verify omitempty omits the key
• strategy/manual_commit_test.go — End-to-end condensation test: SaveChanges → user commit → CondenseSession → verify commit_tree_hash matches HEAD's tree hash

What is NOT changed

• No read paths, CLI commands, or display logic — field is write-only for now
• No changes to temporary checkpoints (shadow branches)
• No migration needed — omitempty means old checkpoints deserialize cleanly

Test plan

• go test ./cmd/entire/cli/checkpoint/ -run TestWriteCommitted_CommitTreeHashField -v
• go test ./cmd/entire/cli/strategy/ -run TestCondenseSession_IncludesCommitTreeHash -v
• gofmt -s -w . — no formatting issues
• golangci-lint run — 0 issues
• go test -tags=integration -race ./... — all tests pass

🤖 Generated with Claude Code