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>

docs/designs/SIDEBAR_MESSAGE_FLOW.md

@@ -71,8 +71,9 @@ T+1-3s    Extension loads, sidebar opens
 
 T+ready   tryAutoConnect calls connect()
             ├── POST /pty-session (Authorization: Bearer AUTH_TOKEN)
-            │   └── server mints session token, posts /internal/grant to agent
-            │   └── responds with {terminalPort, ptySessionToken}
+            │   └── server mints attach token, posts /internal/grant to agent
+            │   └── responds with {terminalPort, sessionId, attachToken,
+            │                      leaseExpiresAt}
             ├── GET /claude-available (preflight)
             ├── new WebSocket(`ws://127.0.0.1:<terminalPort>/ws`,
             │                 [`gstack-pty.<token>`])