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/gbrain-sync.md

@@ -31,7 +31,7 @@ it; you can append your own entries below the marker line.
 ## First-run setup (30–90 seconds)
 
 ```bash
-gstack-brain-init
+gstack-artifacts-init
 ```
 
 The command:
@@ -41,7 +41,7 @@ The command:
    gstack-brain-$USER`). Any git remote works — GitHub, GitLab, Gitea,
    self-hosted.
 3. Pushes an initial commit with just the config.
-4. Writes `~/.gstack-brain-remote.txt` (URL-only, no secrets —
+4. Writes `~/.gstack-artifacts-remote.txt` (URL-only, no secrets —
    safe to copy to another machine).
 5. Wires the gstack-brain repo into your local gbrain as a federated
    source (via `gbrain sources add` + `git worktree`) so `gbrain search`
@@ -65,14 +65,15 @@ Your answer is persisted. You won't be asked again.
 
 ## Cross-machine workflow
 
-On machine A: run `gstack-brain-init` once. That's it — every skill
+On machine A: run `gstack-artifacts-init` once. That's it — every skill
 invocation now drains the sync queue at its start and end boundaries
 (~200–800 ms network pause per skill).
 
 On machine B:
 
-1. Copy `~/.gstack-brain-remote.txt` from machine A to machine B
-   (password manager, dotfile repo, USB stick — your call).
+1. Copy `~/.gstack-artifacts-remote.txt` from machine A to machine B
+   (password manager, dotfile repo, USB stick — your call; the legacy
+   `~/.gstack-brain-remote.txt` name is still recognized).
 2. Run any gstack skill. The preamble sees the URL file and prints:
    ```
    BRAIN_SYNC: brain repo detected: <url>
@@ -180,7 +181,7 @@ This:
 Add `--delete-remote` to also delete the private GitHub repo (GitHub only,
 uses `gh repo delete`).
 
-Re-init anytime with `gstack-brain-init`.
+Re-init anytime with `gstack-artifacts-init`.
 
 ## Troubleshooting
 
@@ -189,8 +190,8 @@ error message gstack-brain may print, with problem / cause / fix for each.
 
 ## Under the hood
 
-For the architectural decisions behind this feature (allowlist vs
-denylist, daemon vs preamble-boundary sync, JSONL merge driver, privacy
-stop-gate), see the
-[approved plan](../system-instruction-you-are-working-jaunty-kahn.md) in
-the gstack plans directory.
+The architectural decisions behind this feature: allowlist over denylist
+(unknown files stay local by default), preamble-boundary sync over a daemon
+(no background process to babysit), a JSONL merge driver so concurrent
+machines union their queues instead of conflicting, and a privacy stop-gate
+that asks once before anything syncs.