feat(mcp): exponential backoff for comment-inbox polling

[scott]

Summary

• Persist MCP inbox poll state in inbox-poll.json (profile dir, mode 0600, atomic rename).
• gander share and hosted gander watch open a 2-hour idle window and reset the interval to 1m. Local preview does not touch the file.
• Gate only no-path MCP gander_list_comments: skip until next_check_at, return {inbox:[], poll:{done:true}} after the window, otherwise GET /api/shares with If-None-Match.
• Backoff 1m → 2m → 4m → 8m on 304 or an unchanged snapshot; reset to 1m and slide the window on count-up or a new uuid. Count-down does not reset.
• Additive poll object on no-path JSON (interval, interval_seconds, next_check_at, stop_at, unchanged, skipped, done).
• Rewrite mcpInstructions to /loop 1m + poll.interval / poll.next_check_at / poll.done. Path-scoped list, reply/resolve, and gander comments still bypass backoff.

Related Issue

Closes #108

Testing

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

Backoff math, skipped/done short-circuit (zero list calls), If-None-Match + 304, share/watch window bump, and mcpInstructions cadence assertions are covered.

Notes

• If the server has no ETag yet, every 200 is treated as a full list (gandermd#114 should deploy first or with this release).
• Skill copy is a paired follow-up (docs: exponential backoff for comment inbox polling gander-skill#16) and must land in the same cycle.
• User-facing gander.md docs are gandermd#115, after this copy is locked.