docs: fold the Codex accuracy review of the release docs

Six findings, all verified against source before fixing:

1. 'Every send writes a receipt' overclaimed — fail-open sinks proceed with
   a stderr warning when the receipt write fails, so a fail-open send can go
   unrecorded (lib/egress-receipt.ts:8-14). Descriptive prose now says so;
   the receipted framing keeps 'attempted'.
2. 'Receipts hash the request body' is wrong for subprocess-owned sends —
   git pushes record sha256: null (lib/egress-receipt.ts:71).
3. 'grants shows every consent in force' overclaimed — it reports the four
   standing config settings (bin/gstack-egress:139-181). Reworded in
   README, ARCHITECTURE, and the CHANGELOG entry.
4. 'Zero-exception scanner' vs reality: the new-sink scanner carries a
   reasoned SCANNER_EXEMPT list (user-directed fetches, probes, instruction
   strings, skill prose). CLAUDE.md now names it.
5. Error-catalog cause/fix for the receipt refusal: the writer mkdirs the
   ledger dir itself, so 'missing' isn't a cause and bare chmod fails when
   it is absent — cause reworded, fix is mkdir -p && chmod.
6. gbrain-sync first-run steps described the retired binary's behavior:
   default repo is gstack-artifacts-$USER, and init PRINTS the gbrain
   hookup command (never auto-executes; bin/gstack-artifacts-init:384-419).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -53,7 +53,7 @@ bug fix and the port shortlist were selected and hardened for upstream.
 #### Added
 - `gstack-egress` — read the hash-chained egress receipt ledger: `list` (what
   gstack attempted to send off-machine), `verify` (recompute the chain, exit 3 on
-  tamper), `grants` (every consent grant in force and how to revoke it).
+  tamper), `grants` (the standing consent settings and how to revoke each).
 - `gstack-context-bill` — offline token bill-of-materials for a skills tree:
   always-on discovery cost vs per-invocation cost, `--diff` between two trees,
   `--budget`, and `--exact` (opt-in, measures against the real tokenizer).