docs: an RFC-style SPEC.md for the whole product

CLAUDE.md

@@ -64,6 +64,12 @@ pending reviews, no comments, no reactions — reviewing is read-only.**
 rules, filing/archiving, and the re-review guards. Keep it true when you change
 any of them — it names the files each rule lives in.
 
+`SPEC.md` is the RFC-style specification of the whole product — schemas,
+lifecycle, tool policy, the send path, in normative language. A PR that
+changes behavior the spec describes updates the spec in the same PR; a
+divergence you find but don't fix goes in its Appendix B rather than being
+left unwritten. Code and tests win when they disagree.
+
 
 - `src/core/` — artifact schema (zod, versioned — the contract between AI and
   cockpit), state store (`~/.cerber/reviews/*.json`, plain JSON, no DB),

README.md

@@ -391,6 +391,12 @@ above: the seven statuses and what sets each, which tab a PR lands in and why,
 when the poll files or archives a row on its own, when a re-review happens and
 what it will not overwrite, and what gets written to disk at each step.
 
+[**SPEC.md**](SPEC.md) is the full service specification — the artifact
+schema, every lifecycle and filing rule, the runner's tool policy and
+credential hygiene, the send path — written precisely enough to reimplement
+cerber from. lifecycle.md is the operator's view of the product; SPEC.md is
+the engineer's.
+
 ## Development
 
 ```bash