feat(memory): review feedback quorum before promotion

**Context:** ROADMAP.md → Memory security → Review feedback quorum
**Related:** #27

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## Component

Agent (Python runtime)

## Describe the feature

Promote review-derived rules to **persistent memory** only after **corroboration**—e.g. pattern seen across trusted reviewers and multiple PRs—reducing single-comment poisoning.

## Use case

One malicious or mistaken review comment should not become a permanent repo rule. Teams want consensus before agent behavior changes.

## Proposed solution

1. Staging area for extracted rules (pending quorum).
2. Quorum config: min reviewers, min PRs, trusted reviewer list.
3. Promote to semantic memory when threshold met; else expire.
4. Operator API to force-promote or reject pending rules.

## Other information

- Complements **Review feedback memory loop** (#27).
- Design context: `docs/design/MEMORY.md`.

- [ ] This might be a breaking change

Start with docs/design/MEMORY.md and the “Memory security” section of ROADMAP.md, then review the related issue #27 for existing memory-loop context. Done means the design and implementation cover pending rule staging, configurable corroboration thresholds, promotion or expiry, and operator force-promote or reject actions.

Written by the indexing model from the issue text.