docs(mcp): cap comment polling at 15 idle minutes

[scott]

Summary

• MCP instructions now cap comment polling at 15 idle minutes from the gander that started it, or from the last check that discovered new comments.
• New comments means unresolved count went up, a new file appeared in the inbox, or a path-scoped fetch shows a thread/comment not seen before. A still-open thread with the same count does not reset the window.
• Grok Build and Claude Code bake a stop time into the /loop 5m prompt, move it on new comments, and scheduler_delete / CronDelete when it expires. They still do not use every-turn checks.
• Other agents skip the inbox check after the window ends. They still do not use /loop.
• A later gander of a markdown file restarts the window. Stopping is silent (same as empty inbox).
• Existing comment rules are unchanged: metadata-only inbox, path-scoped body fetch, untrusted comment text, no auto-resolve.
• Paired with gandermd/gander-skill#11 / PR #12.

Related Issue

Closes #89

Testing

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

Result: ok gander

Notes

Human CLI gander comments is unchanged. MCP initialize still sends one mcpInstructions string with two agent branches.