Archive docs/agents/runs session dumps

[AustinKelsay]

Part of #154
Blocked by: none

Goal

Move pre-ledger session, review-packet, and CodeRabbit dump files under docs/agents/runs/archive/, keep durable *-ledger.md / final-audit files in place, and add docs/agents/runs/README.md indexing what remains.

Acceptance

• Session/review/coderabbit dumps live under archive/.
• Ledgers and final audits remain at docs/agents/runs/.
• README explains the layout and points at current ledgers including the Tier A+B ledger.
• git history preserves moves (git mv preferred).

Notes

AFK. Docs-only. Do not delete evidence.

[AustinKelsay]

Implemented on feature/cleanup-tier-ab-api-surface and included in PR #163 targeting staging. Spec #154.