fix(browse): extension token bootstrap moves to pinned-origin POST; /health carries no token

GET /health is now liveness/status only in every mode — both token
carve-outs (headed-mode disjunct AND chrome-extension:// Origin
disjunct) are removed. Token bootstrap is POST /extension-token on the
local listener: the Origin header must be exactly
chrome-extension://<GSTACK_EXTENSION_ID> and the Host header's hostname
must parse to 127.0.0.1 or localhost (parsed via new URL, never literal
equality — Host arrives as '127.0.0.1:34567'). Wrong origin/host → 403
with no detail. The tunnel surface 404s the endpoint (not in
TUNNEL_PATHS, verified by test).

The extension ID is pinned by a new "key" field (RSA public key) in
extension/manifest.json; browse/scripts/extension-id.ts reproduces the
ID derivation (first 16 bytes of SHA-256 of the DER public key, hex
mapped 0-9a-f → a-p). The private key is not committed anywhere —
unpacked/baked-in loads only need the public key.

Extension side: background.js bootstraps and refreshes the token via
POST /extension-token (403 → disconnected state); sidepanel.js direct
connect path does the same; sidepanel-terminal.js's dead /health token
fallback (read AUTH_TOKEN/authToken keys the server never sent,
hardcoded port) is replaced with the window.gstackAuthToken path.

MIGRATION NOTE: the manifest key pins the extension ID, so existing
installs' side-panel local state (saved port, snoozes) resets once —
explained in-product via a one-time notice (flag
gstack_id_migrated_v162). After upgrading the server, restart the
browser so the old service worker stops polling for a token GET /health
no longer serves.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
(cherry picked from commit e9a0b6847a2d17fe6656a4686b4efd0c8380eb09)

docs/REMOTE_BROWSER_ACCESS.md

@@ -56,7 +56,7 @@ All command endpoints require a Bearer token:
 Authorization: Bearer gsk_sess_...
 ```
 
-`/connect` is unauthenticated (rate-limited) — it's how a remote agent exchanges a setup key for a scoped session token. `/health` is unauthenticated on the local listener (bootstrap) but does NOT exist on the tunnel listener (404).
+`/connect` is unauthenticated (rate-limited) — it's how a remote agent exchanges a setup key for a scoped session token. `/health` is unauthenticated on the local listener (liveness/status only — never a token) but does NOT exist on the tunnel listener (404). Extension token bootstrap is `POST /extension-token` on the local listener, gated by the pinned `chrome-extension://` Origin; it is not on the tunnel surface either.
 
 SSE endpoints (`/activity/stream`, `/inspector/events`) accept either a Bearer token or the HttpOnly `gstack_sse` cookie (minted via `POST /sse-session`, 30-minute TTL, stream-scope only — cannot be used against `/command`). As of v1.6.0.0 the `?token=<ROOT>` query-string auth is no longer accepted.
 
@@ -80,6 +80,9 @@ Response: (plain text result of the command)
 
 #### GET /health
 Server status. No auth required. Returns status, tabs, mode, uptime.
+Never carries a token — extension token bootstrap is `POST /extension-token`
+(local listener only, validates the pinned `chrome-extension://` Origin and a
+loopback Host; 403 otherwise). Not reachable over the tunnel (404).
 
 ### Commands
 

docs/designs/SIDEBAR_MESSAGE_FLOW.md

@@ -62,6 +62,10 @@ T+500ms   terminal-agent.ts boots
             └── Probes claude → writes claude-available.json
 
 T+1-3s    Extension loads, sidebar opens
+            ├── background.js: GET /health (liveness only — no token) then
+            │   POST /extension-token → AUTH_TOKEN. The server releases the
+            │   token only to Origin chrome-extension://<pinned id>; the
+            │   manifest "key" pins the ID (browse/scripts/extension-id.ts)
             ├── sidepanel-terminal.js: setState(IDLE), shows "Starting Claude Code..."
             └── tryAutoConnect() polls until window.gstackServerPort + token are set
 
@@ -105,7 +109,7 @@ The protocol-token path is what the browser actually uses.
 
 | Token | Lives in | Used for | Lifetime |
 |-------|----------|----------|----------|
-| `AUTH_TOKEN` | `<stateDir>/browse.json`; in-memory in server.ts | `/pty-session` POST (mint cookie + token) | server lifetime |
+| `AUTH_TOKEN` | `<stateDir>/browse.json`; in-memory in server.ts; extension memory via pinned-origin `POST /extension-token` (never `GET /health`) | `/pty-session` POST (mint cookie + token) | server lifetime |
 | `gstack-pty.<...>` (Sec-WebSocket-Protocol) | Browser memory only; agent `validTokens` Set | `/ws` upgrade auth | 30 min, auto-revoked on WS close |
 | `INTERNAL_TOKEN` | `<stateDir>/terminal-internal-token`; in agent memory | server → agent loopback `/internal/grant` | agent lifetime |