feat(phase34): Git worktrees and branch sandboxing with /branch command and subagent

[De-pitcher]

Phase 34 Summary: Git Worktrees & Branch Sandboxing

🎯 Key Capabilities Added

1. WorktreeManager (\�gentcli.worktree.manager):
   • Isolated Git worktree creation under .agentcli/worktrees/\ with base ref branching.
   • Live worktree discovery, status checking (dirty detection, modified files listing), unified diff computation (\compute_diff), merge (\squash\ or
     o-ff), safe removal, and orphan pruning.
   • Windows file handle safety and clean metadata persistence in .worktrees.json.
2. Worktree SubAgent (\�gentcli.subagents.worktree):
   • Registered as \SubAgentType.WORKTREE\ (\worktree) in \ToolRegistry\ and \TOOL_DEFINITIONS.
   • Supports autonomous actions: \create, \list, \status, \diff, \merge, \discard, \prune.
3. Session Grounding:
   • Attached \session.worktree_manager = WorktreeManager()\ and workspace sandbox tracking in \�gentcli.session.
4. Slash Command & TUI Integration:
   • /branch [list | create | status | diff | merge | discard | prune]\ with aliases /worktree\ and /wt.
   • Autocompletion in \SlashAndFileCompleter\ and action handling in Textual TUI dashboard.

🛡️ Quality Gates & Verification

• Unit & Integration Tests: 13/13 Phase 34 tests passed, 103/103 core suite passed.
• Ruff Linting: 0 errors.
• Mypy Type Checking: 0 errors across 124 source files.