fix(inventory): rfcs now HAS a BUGBOT.md, so the exemption is stale

[LukasWodka]

The fleet audit is red on develop with exactly one finding:

rfcs: .cursor/BUGBOT.md is marked \exempt` but the file exists. The exemption is stale — promote it to `required`.`

rfcs#23 merged at 07:04 today and added the file. The exemption's own text said "UNREMEDIATED — this exemption is the finding, not a justification" — true when written, and it stopped being true the moment someone closed it. The guard catching its own stale exemption is the family working as designed.

Why it matters beyond the repo it names

caller-drift runs on every PR that touches the guard or the inventory, so a single stale exemption reddens all of them. .github#255 is held by it right now, and its own change is unrelated.

The shared reason goes with it

bugbot_guide_missing was cited by rfcs alone. Promoting the entry left an anchor with zero users, and shared_reasons is defined in this file as "reasons cited by more than one repo". A shared reason that nothing shares is the same class of stale artefact as the exemption itself.

Verification

• caller-drift-selftest.py — 162/162 pass
• YAML parses; rfcs.quality_files now reads CLAUDE.md: required, .cursor/BUGBOT.md: required
• .cursor/BUGBOT.md confirmed present in rfcs (8843 bytes)
• The full-fleet audit was not re-run to completion locally — it exceeded a 7-minute budget against a rate-limited token. CI's required audit job is the confirmation, not this description.

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Note

Low Risk
Contract-only YAML in the fleet inventory; no runtime, auth, or deployment behavior changes.

Overview
rfcs now ships .cursor/BUGBOT.md, but repo-inventory.yml still marked that path exempt via bugbot_guide_missing, so caller-drift reported a stale exemption and blocked unrelated PRs that touch the inventory.

This PR promotes rfcs quality_files[".cursor/BUGBOT.md"] to required and drops the unused bugbot_guide_missing anchor from shared_reasons (it was cited only by rfcs).

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