feat(privacy): audited redaction that keeps history's shape and removes its content

Rows in append-only history are never deleted; their content can now be
made irrecoverable through one audited path. A redaction replaces the
content columns of an event, memory, message, model call, belief or
assertion with a fixed marker, keeps identifiers, links, timestamps,
scores and breakdowns, writes an append-only ledger row with a SHA-256 of
the removed content, appends a redaction event, optionally cascades to
derived rows and to the events that quote them, and scrubs stored snapshot
archives before re-signing them. noema_redact() is the only write the
append-only trigger admits, for UPDATE only, under a transaction-local flag
the application never sets; plain updates and all deletes stay refused.
Replay labels redacted stimuli instead of inventing them.

Cryptographic erasure was considered and deferred with reasons (ADR-0009);
docs/privacy.md defines retained, redacted, irrecoverable and deleted and
states what backups and earlier exports are not covered. Migration 0023.

CHANGELOG.md

@@ -10,6 +10,7 @@ All notable changes to Noema are recorded here. Format follows Keep a Changelog;
 - Security suite: SQL-injection effect test, static allow-list scan for assembled SQL, deceived-mind authority test, garbage model output test, self-preservation vocabulary test; threat model rewritten so every verification cites a test that asserts the claim.
 
 ### Added
+- Privacy redaction (ADR-0009): the audited path that makes an event, memory, message, model call, belief or assertion irrecoverable while keeping the row, its links, timestamps and scores; a ledger with a hash of the removed content; cascade to derived rows and the events that quote them; stored snapshot archives scrubbed and re-signed; replay labels redacted stimuli. `noema_redact()` is the only write the append-only trigger admits. Migration 0023. `docs/privacy.md` defines retained, redacted, irrecoverable and deleted.
 - Causal timeline: `GET /api/v1/events/{id}/chain` and `/events/{id}/timeline` walk the links events actually record (causation, parents, cited evidence) backwards and forwards within bounded depth and size, label edge kinds, group by cycle as correlation only, and infer nothing.
 - Cognitive forks per model: minds may override model routing per role (`llm_roles`, fail-closed); experiments may declare variants that fork one snapshot under different configuration, personality or routing, receive identical stimuli, and are compared deterministically (first divergent stimulus, belief and attention overlap, score differences, memory, goal, cost and capability-request ranges).
 - Runtime identity: snapshots, replays and experiments record build (commit, modified tree, Go version, platform), schema version, a secret-free configuration digest, personality version, policy digest, model routing, prompt versions, deterministic subsystem versions and timezone. Replays compare identities and label themselves exact-candidate, different-build or incompatible (the last is never run), and analyse the first divergence by component. Migration 0022.