chore: run review-memory weekly and keep lone inbox notes one review

[V3RON]

The review-memory skill can now run on a weekly schedule without losing notes. Until now it emptied the whole inbox after every run, so a note only got one run to find a matching note before being promoted. That made its own "sat alone for two reviews" rule impossible. Now a note that doesn't match anything is kept with a Seen: <date> line and dropped on its second review. That gives notes two weeks to pair up on a weekly schedule.

The skill also stops without creating a branch or PR when there's nothing to review (an empty inbox and no section over its cap) or when an earlier memory PR is still open. Mechanism issues skip the file-issue interview, since the notes already are the spec. AGENTS.md and the inbox header now say weekly instead of monthly.

With this merged, the weekly routine prompt shrinks to:

Run the review-memory skill (.claude/skills/review-memory/SKILL.md). This run is unattended: never stop to ask. Where the skill wants a human, keep the entry rather than drop it and leave the PR open rather than merge it, and say why in the report.

Not user-visible, so there's no changelog entry and no E2E evidence.

Out of scope

The memory/<date> branch name may need a claude/ prefix if cloud routines can only push those branches. Worth checking on the first scheduled run.