# T0.3: Git & Locking

## Scope
Implement `action/git.go` + `internal/lock/flock.go` for repo cloning, worktree management, and squash-merge.

## Implementation

### File: `internal/lock/flock.go`
```go
package lock

// Acquire advisory file lock (blocking)
func Acquire(path string) error

// Release advisory file lock
func Release(path string) error
```

### File: `action/git.go`
```go
type CloneRepoInput struct {
    RemoteURL      string
    TargetRepoPath string
}

func CloneRepoActivity(ctx context.Context, in CloneRepoInput) error
    // If $TargetRepoPath/.git exists: git -C $TargetRepoPath fetch origin
    // Else: git clone $RemoteURL $TargetRepoPath

type GitWorktreeAddInput struct {
    RepoPath string
    TaskID   string
}

func GitWorktreeAddActivity(ctx context.Context, in GitWorktreeAddInput) (string, error)
    // Guarded by orchestrator.lock
    // git worktree add -b task/<TaskID> ../worktrees/<id> origin/main
    // Return worktree path

type GitCommitInput struct {
    WorktreePath string
    Message      string
}

func GitCommitActivity(ctx context.Context, in GitCommitInput) error
    // No lock needed; safe within isolated worktree
    // git -C $WorktreePath add -A
    // git -C $WorktreePath commit -m "$Message"

type GitPushInput struct {
    RepoPath string
}

func GitPushActivity(ctx context.Context, in GitPushInput) error
    // Guarded by orchestrator.lock
    // git -C $RepoPath push origin main

type GitSquashMergeInput struct {
    RepoPath string
    Branches []string // ["task/T0.1", "task/T0.2", ...]
    Message  string
}

func GitSquashMergeActivity(ctx context.Context, in GitSquashMergeInput) error
    // Guarded by orchestrator.lock
    // fetch origin main
    // checkout main && pull --ff-only origin main
    // for b in branches: merge --squash $b
    // commit -m $Message
    // push origin main
    // for b in branches: worktree remove + branch -D
```

## Verification
```bash
cd /Users/rockliang/workplace/Poimen/workflows
go test -v ./tests -run TestGit

# Test script: tests/git_test.go
```

Test cases:
- Clone into empty path → creates .git
- Clone into existing path → fetches instead of re-cloning
- Worktree add → returns valid path
- Commit in worktree → file changes staged
- Squash-merge → one commit on main, branches cleaned up

## Done Criteria
- `go test ./tests -run TestGit` passes
- Tested against local scratch git repo (not real remote)
- No lock deadlocks on concurrent calls
- Squash-merge produces exactly one commit