[eric] docs: add the secret rotation + history-purge runbook so the leaked creds can actually be killed

docs/SECRET_ROTATION.md

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+# Secret rotation + history purge
+
+The repo history contains real credentials that were committed long ago (the
+ones `.gitleaksignore` acknowledges). CI is green because those findings are
+allowlisted, but **allowlisting hides them, it does not remove them** — they're
+still in `git log` and still shipped in the app today. This is the real fix.
+
+> Requires repo **admin** + access to the provider consoles + a **force-push**
+> (history rewrite). The agent can't do any of those, so this is a human runbook.
+
+## 1. Rotate first (this is what actually kills the exposure)
+
+Rotating invalidates the leaked value immediately, so even though it stays in
+history it becomes useless. Do this before bothering with the purge.
+
+| Secret | Where it leaked (commit) | Rotate where |
+|--------|--------------------------|--------------|
+| Google OAuth client secret | `backend/apps/tools_lib/{oauth_providers,tools_lib}.py` (7239f70, 7c3da1a, cbefe89) | Google Cloud Console → APIs & Services → Credentials → the OAuth client → **Reset secret** |
+| PostHog API key | `backend/apps/analytics/collector.py` (8d09e46, b6f45e8) | PostHog → Project settings → rotate project API key (note: ingest keys are public by design — rotate only if it's a private key) |
+| 9router client secrets | `9router/**` (cf775b4, history-only; dir now fetched from npm) | Whichever provider each `clientSecret` belongs to; bump `ROUTER_VERSION` if the npm package itself shipped one |
+
+After rotating, update wherever the build injects them (the `GOOGLE_OAUTH_*`
+GitHub Actions secrets + `backend/.env` production-injection step) to the new
+values, and cut a release so users get the rotated build.
+
+## 2. Purge from history (optional, after rotation)
+
+Redact the values from every commit with [git-filter-repo](https://github.com/newren/git-filter-repo):
+
+```bash
+# expressions.txt: one `OLD_SECRET==>REDACTED` per line (the real old values)
+git filter-repo --replace-text expressions.txt
+```
+
+Then the destructive part (admin only):
+
+- `git push --force --all` and `git push --force --tags` (this is why the agent
+  can't do it — force-push is denied and it rewrites every downstream commit hash).
+- Everyone re-clones (old clones still hold the secrets).
+- Re-create any protected-branch/tag rulesets if the rewrite trips them.
+
+Because rotation (step 1) already neutralizes the secret, the purge is about
+hygiene, not urgency. Once both are done, drop the matching fingerprints from
+`.gitleaksignore`.