docs: sweep drift that predates this release

Surfaced by the /document-release audit; every fix verified against the
current binaries:

- gstack-brain-init was replaced by gstack-artifacts-init in v1.27.0.0
  (hard-delete, no compat shim), but README, USING_GBRAIN_WITH_GSTACK,
  docs/gbrain-sync.md, and docs/gbrain-sync-errors.md still instructed
  users to run it — command-not-found on every follow. Same sweep updates
  ~/.gstack-brain-remote.txt to the canonical ~/.gstack-artifacts-remote.txt
  (legacy name still honored on restore, noted where users copy the file).
- gbrain-sync-errors.md headings re-matched to the literal messages the
  binaries print today (the doc's whole value is grep-by-exact-message):
  'gstack-artifacts-init: ~/.gstack/ is already a git repo pointing at:',
  'Remote not reachable via SSH:', 'Failed to create or find ...'. The
  already-a-repo fix now leads with the command's own set-url suggestion.
- docs/gbrain-sync.md 'Under the hood' linked a plan file that does not
  exist in the repo; replaced with the decisions themselves.
- SIDEBAR_MESSAGE_FLOW startup timeline: /pty-session responds with
  {terminalPort, sessionId, attachToken, leaseExpiresAt} (v1.44 shape,
  verified at browse/src/server.ts:1860), not the retired
  {terminalPort, ptySessionToken} pair.

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

README.md

@@ -420,7 +420,7 @@ The skill asks once per repo. The decision is sticky across worktrees and branch
 **GStack memory sync (different feature, same private-repo infra).** Optionally pushes your gstack state (learnings, CEO plans, design docs, retros, developer profile) to a private git repo so your memory follows you across machines, with a one-time privacy prompt (everything allowlisted / artifacts only / off) and a defense-in-depth secret scanner that blocks AWS keys, tokens, PEM blocks, and JWTs before they leave your machine.
 
 ```bash
-gstack-brain-init
+gstack-artifacts-init
 ```
 
 **Running gstack in Conductor?** Conductor explicitly strips `ANTHROPIC_API_KEY` and `OPENAI_API_KEY` from every workspace's process env, so paid evals and gbrain embeddings won't work out of the box. Set `GSTACK_ANTHROPIC_API_KEY` and `GSTACK_OPENAI_API_KEY` in Conductor's workspace env config instead — gstack's TS entry points promote them to canonical names at runtime. Full details and the contributor checklist for adding the import to new entry points: [Conductor + GSTACK_* env vars](USING_GBRAIN_WITH_GSTACK.md#conductor--gstack_-env-vars).