Add testable task descriptions: T0.1 through T0.9

Each task includes:
- Scope: what to build
- Implementation: code sketches + details
- Verification: concrete test criteria
- Done criteria: acceptance checklist

tasks/T0.3.md

@@ -0,0 +1,93 @@
+# 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