#22662 cmux-next sidebar: the Lanes item (every agent and its state) shows without Labs

Leo: "What are all these agents? I'm not sure which ones are actually running, which ones are actually doing anything... Isn't there an expanded view of things?"

There is one. The lanes view of the Agent Activity page (cx-v9k2f) lists every agent chat with the state acpmux decides for it (working, needs input, failed with its error text such as a usage limit, lost, idle), what it's doing now (the lane's step), since when, and its workspace. Clicking a row opens that chat. The sidebar has a Lanes item at the top that opens it and counts the chats that need a person. Both showed only while Labs preview features were on, which is off by default, so Leo never saw them.

Change

• SidebarWhatsNewItemFeed: the Lanes item shows whenever a lane shows, with or without Labs.
• AgentActivityPageService / AgentLaneStore.follow: the page's lanes view always follows the lanes, with no Labs gate.
• Unchanged: Leo's 2026-10-15 rule that the item counts this Mac's own sessions, not peers' fleet lanes or a coordinator's child sessions, unless agentActivity.lanesShowFleet (Debug Settings) is on. Turning that toggle on shows his sub-sessions too.

The Labs gate's comment said "until the lanes view is styled". Leo's request decides it here; say if you'd rather keep the gate.

Commits: the first updates AgentLanesTests to expect the item without Labs (red), the second removes the gate.

Not in this PR

• Chief subagents (a1…a14) run on the Chief's own acpmux. Their lanes go to a separate AgentLaneStore (ChatsFeed.startChiefTurnStates), so the lanes view doesn't list them yet.
• No new styling.

Verification

AgentLanesTests.theLanesItemShowsWithoutLabsPreviewFeatures runs on CI (the Swift test job is blocked on base's web-bundles red, #22649). Not run locally (no Mac builds on this machine). Not dogfooded.

Agent session: cosmic-gray

🤖 Generated with Claude Code

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Summary by cubic

Makes the Lanes sidebar item and the Agent Activity lanes view available by default, so the expanded view of every agent and its state shows without Labs preview features.

• The lanes view lists each agent chat with its state (working, needs input, failed with its error, lost, idle), its current step, and since when; clicking a row opens that chat.
• The Lanes item still shows only while a lane exists, counts the chats that need a person, and keeps the rule that it counts this Mac's own sessions unless agentActivity.lanesShowFleet is on.
• Chief subagents' lanes are still not listed because they run on their own AgentLaneStore.