Apply P14: the brief's State list names brokerd's session state

broker/sessions/<id>.json holds each session's taint and untrusted
flag. It is written only by brokerd and can be rebuilt from the audit
log's result records.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/specs/2026-09-18-m3a-decision-path.md

@@ -296,7 +296,7 @@ pub fn redecide(ask: Ask, grants: &GrantSet, state: SessionState, now: Timestamp
 - Writes are atomic: write `<id>.json.tmp`, `fsync`, rename over `<id>.json`, `fsync` the directory.
 - A file that exists but cannot be read or parsed is an error: every call for that session is
   denied with `state_unreadable` and `brokerd` prints `see docs/runbook.md#broker-state-damaged`.
-- The brief's State list does not name this directory yet; P14 in `docs/decisions.md` proposes it.
+- The brief's State list names this file (P14, applied 2026-09-18).
 - `loopd` has no access to `<home>/broker/`. The `class` and `untrusted` values it logs are a copy
   for its own use and never an input to policy.