WorktreeDirty error contains CLI-specific advice

[wollax]

Problem

WorktreeDirty error message says "use --force to override" which is CLI-specific advice. When this error surfaces through MCP (where force defaults to true), the message is confusing and irrelevant.

Solution

Solution

Make the domain error message context-neutral (e.g., "worktree {spec_slug} has uncommitted changes") and let the CLI layer append the --force hint when displaying the error.

---

Created via Kata PR review backlog