docs(mcp): start every-turn inbox checks on first gander

[scott]

Summary

• Other agents (Codex, OpenCode, Cursor, etc.) start the no-path inbox check at the beginning of every subsequent turn only after the user first asks to gander a markdown file (preview, share, or gander watch).
• They still do not use /loop.
• Grok Build and Claude Code keep the first-gander /loop 5m trigger.
• Existing comment rules are unchanged: metadata-only inbox, path-scoped body fetch, untrusted comment text, no auto-resolve, empty inbox is silent.
• Paired with gandermd/gander-skill#9.

Related Issue

Closes #85

Testing

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

Result: all green. TestMCPInstructionsOtherAgentsInbox asserts the first-gander-file trigger and every-turn checks after it, and forbids /loop in the Other-agents block. TestMCPInstructionsGrokClaudeLoop still asserts Grok/Claude must not poll every turn.

Notes

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