feat(extension): Terminal-only sidebar — auth fix, UX polish, chat rip

The chat queue path is gone. The Chrome side panel is now just an
interactive claude PTY in xterm.js. Activity / Refs / Inspector still
exist behind the `debug` toggle in the footer.

Three threads of change, all from dogfood iteration on top of
cc-pty-import:

1. fix(server): cross-port WS auth via Sec-WebSocket-Protocol
   - Browsers can't set Authorization on a WebSocket upgrade. We had
     been minting an HttpOnly gstack_pty cookie via /pty-session, but
     SameSite=Strict cookies don't survive the cross-port jump from
     server.ts:34567 to the agent's random port from a chrome-extension
     origin. The WS opened then immediately closed → "Session ended."
   - /pty-session now also returns ptySessionToken in the JSON body.
   - Extension calls `new WebSocket(url, [`gstack-pty.<token>`])`.
     Browser sends Sec-WebSocket-Protocol on the upgrade.
   - Agent reads the protocol header, validates against validTokens,
     and MUST echo the protocol back (Chromium closes the connection
     immediately if a server doesn't pick one of the offered protocols).
   - Cookie path is kept as a fallback for non-browser callers (curl,
     integration tests).
   - New integration test exercises the full protocol-auth round-trip
     via raw fetch+Upgrade so a future regression of this exact class
     fails in CI.

2. fix(extension): UX polish on the Terminal pane
   - Eager auto-connect when the sidebar opens — no "Press any key to
     start" friction every reload.
   - Always-visible ↻ Restart button in the terminal toolbar (not
     gated on the ENDED state) so the user can force a fresh claude
     mid-session.
   - MutationObserver on #tab-terminal's class attribute drives a
     fitAddon.fit() + term.refresh() when the pane becomes visible
     again — xterm doesn't auto-redraw after display:none → display:flex.

3. feat(extension): rip the chat tab + sidebar-agent.ts
   - Sidebar is Terminal-only. No more Terminal | Chat primary nav.
   - sidebar-agent.ts deleted. /sidebar-command, /sidebar-chat,
     /sidebar-agent/event, /sidebar-tabs* and friends all deleted.
   - The pickSidebarModel router (sonnet vs opus) is gone — the live
     PTY uses whatever model the user's `claude` CLI is configured with.
   - Quick-actions (🧹 Cleanup / 📸 Screenshot / 🍪 Cookies) survive
     in the Terminal toolbar. Cleanup now injects its prompt into the
     live PTY via window.gstackInjectToTerminal — no more
     /sidebar-command POST. The Inspector "Send to Code" action uses
     the same injection path.
   - clear-chat button removed from the footer.
   - sidepanel.js shed ~900 lines of chat polling, optimistic UI,
     stop-agent, etc.

Net diff: -3.4k lines across 16 files. CLAUDE.md, TODOS.md, and
docs/designs/SIDEBAR_MESSAGE_FLOW.md rewritten to match. The sidebar
regression test (browse/test/sidebar-tabs.test.ts) is rewritten as 27
structural assertions locking the new layout — Terminal sole pane,
no chat input, quick-actions in toolbar, eager-connect, MutationObserver
repaint, restart helper.

TODOS.md

@@ -52,28 +52,6 @@ scope of that PR; deliberately deferred to keep PTY-import small.
 
 ---
 
-### v1.1+: Apply terminal-agent's exception handlers to sidebar-agent
-
-**What:** While reviewing cc-pty-import, codex noted that `sidebar-agent.ts`
-has no `process.on('uncaughtException'|'unhandledRejection')` handlers.
-A bug in claude stream parsing or queue I/O can take down the chat path
-silently. terminal-agent.ts ships with these handlers; sidebar-agent
-should get them too.
-
-**Why:** Today a single uncaught exception in chat = entire sidebar chat
-dies and nothing tells the user. The CLI doesn't supervise the agent.
-
-**Pros:** Chat survives transient bugs. **Cons:** Catching uncaught
-exceptions can hide real failures — pair the handlers with structured
-logging so we still see the bug.
-
-**Context:** codex finding #4 on cc-pty-import plan-eng review.
-
-**Priority:** P2.
-**Effort:** S.
-
----
-
 ## Testing
 
 ### Pre-existing test failures surfaced during v1.12.0.0 ship