feat: add gt crew add command for user-managed workspaces

Implements gt-cik.2: Create crew workspace command with:
- internal/crew/types.go: CrewWorker type definition
- internal/crew/manager.go: Manager for crew lifecycle
- internal/crew/manager_test.go: Unit tests
- internal/cmd/crew.go: CLI command with --rig and --branch flags

Crew workers are user-managed persistent workspaces that:
- Clone repo into <rig>/crew/<name>/
- Create optional feature branch (crew/<name>)
- Set up mail directory for delivery
- Initialize CLAUDE.md with crew worker prompting
- Are NOT registered with witness (user-managed)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/crew/types.go

@@ -0,0 +1,39 @@
+// Package crew provides crew workspace management for overseer workspaces.
+package crew
+
+import "time"
+
+// CrewWorker represents a user-managed workspace in a rig.
+type CrewWorker struct {
+	// Name is the crew worker identifier.
+	Name string `json:"name"`
+
+	// Rig is the rig this crew worker belongs to.
+	Rig string `json:"rig"`
+
+	// ClonePath is the path to the crew worker's clone of the rig.
+	ClonePath string `json:"clone_path"`
+
+	// Branch is the current git branch.
+	Branch string `json:"branch"`
+
+	// CreatedAt is when the crew worker was created.
+	CreatedAt time.Time `json:"created_at"`
+
+	// UpdatedAt is when the crew worker was last updated.
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+// Summary provides a concise view of crew worker status.
+type Summary struct {
+	Name   string `json:"name"`
+	Branch string `json:"branch"`
+}
+
+// Summary returns a Summary for this crew worker.
+func (c *CrewWorker) Summary() Summary {
+	return Summary{
+		Name:   c.Name,
+		Branch: c.Branch,
+	}
+}