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)

browse/src/browser-manager.ts

@@ -1561,8 +1561,8 @@ export class BrowserManager {
       if (extensionPath) {
         launchArgs.push(`--disable-extensions-except=${extensionPath}`);
         launchArgs.push(`--load-extension=${extensionPath}`);
-        // Auth token is served via /health endpoint now (no file write needed).
-        // Extension reads token from /health on connect.
+        // Auth token is served via POST /extension-token (pinned-origin
+        // bootstrap, no file write needed). /health is liveness-only.
         console.log(`[browse] Handoff: loading extension from ${extensionPath}`);
       } else {
         console.log('[browse] Handoff: extension not found — headed mode without side panel');