feat(mcp): ask once before watching a project directory

[scott]

Summary

• Teach MCP-only agents the same one-time folder ask as the skill: when they create a project/output directory (or write the first .md into one that is not already dir-watched), ask before gander watch <dir>.
• Yes runs hosted gander watch <abs-dir> (or local gander --watch <abs-dir> if not signed up) and starts the usual comment-poll window. No is a session skip. Already watching does not ask.
• Never silent auto-watch. File watches still use gander watch --silent <path>. Directory watch is silent by design.

Related Issue

Closes #124

Testing

• go test ./...
• go vet ./...
• go build ./...

Notes

• Ask text matches gander-skill#24 / PR 26.
• Pairs with directory watch gander-cli#123 / PR 126 and docs gandermd#130. Same CLI release.