docs(worktrees): API reference, wiki page and the invariants behind them

Documents the worktree endpoints and quick-start's worktree field in
docs/api-reference.md, adds a Git Worktrees page to the wiki (sidebar,
Home index, a pointer from Core Concepts where sessions sharing one
case folder is explained), and records the rules that keep the feature
safe in docs/architecture-invariants.md with a short CLAUDE.md pointer:
only the managed .claude/worktrees path is ever touched, git never runs
over a Docker workspace, removal is never forced and branch delete is
-d only, and sessions start through the existing create paths.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/wiki/Home.md

@@ -66,6 +66,7 @@ codeman web    # then open http://localhost:3000
 | [Agent CLIs](Agent-CLIs)                   | Which agent should this session run, and how do I set it up? |
 | [Custom Model Endpoints](Custom-Model-Endpoints) | How do I point a session at my own OpenAI-compatible endpoint? |
 | [Working With Files](Working-With-Files)   | How do I read, edit, and attach files?                     |
+| [Git Worktrees](Git-Worktrees)             | How do several agents work on one repository without sharing a folder? |
 | [Input And Voice](Input-And-Voice)         | How do I talk to an agent, including by voice?             |
 | [Mobile Guide](Mobile-Guide)               | How well does this work on a phone?                        |
 | [Keyboard Shortcuts](Keyboard-Shortcuts)   | What can I drive from the keyboard?                        |