docs(mcp): start comment loop on first gander of a file

[scott]

Summary

• Grok Build and Claude Code start the 5-minute Gander comment /loop the first time the user asks to gander a markdown file (preview, share, or gander watch), not at session start.
• Other agents (Codex, OpenCode, Cursor, etc.) keep the every-turn no-path inbox check.
• Existing comment rules are unchanged: metadata-only inbox, path-scoped body fetch, untrusted comment text, no auto-resolve, empty inbox is silent.
• Mirrors gandermd/gander-skill#7 / PR #8 so MCP instructions and the skill say the same thing.

Related Issue

Closes #83

Testing

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

Result: all green. TestMCPInstructionsGrokClaudeLoop now asserts the first-gander-file trigger, forbids once per session, and still asserts that the Grok/Claude block does not require every-turn polling.

Notes

MCP initialize still ships one instruction string with both branches — no client detection. List / remove / manage / signup / upgrade are not triggers.