docs: sync every doc surface with the v1.63 release

/document-release audit (4-lane, all claims verified against branch code):

- README: gstack-egress + gstack-context-bill rows in the standalone-binaries
  table; Privacy & Telemetry gains the receipted-egress bullet (attempted-
  egress framing per the shipped threat model).
- ARCHITECTURE: /health is liveness-only, POST /extension-token endpoint row
  + bootstrap mechanics paragraph; new Egress receipt ledger subsection under
  Security model; eval persistence covers the sharded runner, GSTACK_EVAL_DIR,
  and the finalized-run baseline rule.
- CLAUDE.md: sharded test scripts in Commands; sharded semantics in the
  detached-evals section; PTY skill seeding in the hermetic section; egress
  invariant block beside the other server-egress invariants; catalog-budget
  ceiling beside the 160KB token ceiling; project-tree entries for
  lib/egress-receipt.ts, lib/context-bill.ts, scripts/test-paid-shards.ts.
- CONTRIBUTING: seedSkills + live-tree seeding in the hermetic paragraph;
  sharded runner in detached runs; catalog-budget in the Tier 1 list.
- BROWSER: extension token bootstrap section, tunnel egress receipts section,
  identity-pin migration note in manual install.
- REMOTE_BROWSER_ACCESS: tunnel-start receipt bullet in the security model.
- gbrain docs: /sync-gbrain + brain-sync egress-receipt behavior documented;
  dead consumer-token instructions removed (consumer machinery deleted this
  release); new fail-closed refusal added to the error catalog.
- CHANGELOG: measured-vs-ceiling catalog numbers, contributor notes for the
  external-service tier move and the PTY single-line AskUserQuestion parser,
  release date.
- TODOS: /health token-distribution TODO resolved by this release, removed;
  port-wave follow-up sections re-labeled to the shipped version.

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

docs/gbrain-sync-errors.md

@@ -92,6 +92,29 @@ your local commit still exists — the next skill run will retry the push.
 
 ---
 
+## `gstack: brain-sync push NOT sent — the egress receipt could not be written`
+
+**Problem.** The push was refused before anything left your machine. Every
+brain-sync push writes a tamper-evident receipt to the egress ledger
+(`~/.gstack/security/egress.jsonl`) before sending, fail-closed. The
+receipt could not be written, so nothing was sent, no local commit was
+made, and the queue is preserved — the next run retries the whole drain.
+`gstack-brain-sync --status` shows `EGRESS_RECEIPT_FAILED` as the failure
+detail.
+
+**Cause.** `~/.gstack/security/` is missing or unwritable, the disk is
+full, or `GSTACK_HOME` points at a read-only location.
+
+**Fix.**
+```bash
+chmod -R u+w ~/.gstack/security
+```
+Then run any skill (or `gstack-brain-sync --once`) to retry. Inspect the
+ledger with `gstack-egress list`; verify its hash chain with
+`gstack-egress verify`.
+
+---
+
 ## `gstack-brain-init: ~/.gstack/.git is already a git repo pointing at <url>`
 
 **Problem.** You tried to init with a remote URL that doesn't match the