fix(browser): keep the panel closed when an agent opens a page

The agent's browser.open used to force the context panel open and steal
the active surface, which read as panels opening by themselves. Tab
upserts now take a reveal option: the agent's opener passes reveal: false,
so the tab mounts invisibly (panes are kept alive regardless of
visibility, so agent control still works) while the panel and the active
tab stay exactly as the user left them. Manual opens are unchanged.

CHANGELOG.md

@@ -26,6 +26,7 @@ All notable changes to this project will be documented in this file.
 - Chat: a "Follow new content while streaming" checkbox (Settings → Chat → Streaming, on by default) turns automatic following off entirely; with it off, the scroll-to-bottom pill now appears as soon as the reply grows past the visible area.
 - Command palette: rarely used commands (pin session, copy session ID, multi-run launcher, archived sessions, notes, todos, status, theme) are found by typing but stay off the first screen.
 - Mobile: narrowing a browser window past phone size switches into the mobile layout (and back when widened); the old/new mobile layout setting is gone.
+- Browser: an agent opening a page with the browser tool no longer pops the browser panel open (or switches the surface you're on) — the page loads in the background and the rail is where you peek at it.
 - Usage: the Command Code tile is gone — their official API exposes no usage data, so the tile could only fail.
 - Auth: an expired OpenChamber login is announced within seconds by a banner with a Log in button, instead of being discovered through failing actions. Sending pauses until login, and a conversation that failed to load reloads itself afterwards.
 - Chat: a failed send returns your typed prompt to the input — whatever the reason — instead of losing it to an error toast; a mid-send session switch lands it in that session's draft.