refactor(ui): move routed surfaces and agent-workspace state onto the view scope (RIG-4442)

[rigel-mintaka]

This PR is part of a stack containing 7 PRs:

1. main
2. refactor(ui): extract parseRoute and add ViewScope with useView (RIG-4439) #1652
3. "refactor(ui): move routed surfaces and agent-workspace state onto the view scope (RIG-4442)" (this PR)
4. feat(ui): window layout model, hash sync and session persistence (RIG-4443) #1743
5. feat(ui): view tab strip with keep-alive and open-in-tab links (RIG-4444) #1755
6. feat(ui): W leader chords and palette commands for tabs and panes (RIG-4445) #1770
7. feat(ui): two-pane split with a resizable splitter (RIG-4446) #1780
8. fix(ui): chrome cleanup: drop ADE and usage bar, Session Log naming, pane edges, row re-measure (RIG-4771) #1811

T2 of the in-window tabs and split views record (docs/designs/ui/compass-in-window-tabs/design.md §T2). Stacked on #1652 (T1). There is still one view per window, so nothing changes for the user.

• agent-tabs.ts (new): the tab and split model moves out of store.ts unchanged.
• view-scope.ts: each view owns its agent workspace (tabs, active tab, repo id), keyed on the route's agent. The same agent, or a non-agent route, keeps it; a different agent starts fresh. This is the old agentViewAgentId rule. It adds agentTabs, activeAgentTab, agentSession, openTab, closeTab, splitFocused and workspaceChannel.
• components/ViewHost.tsx (new): renders a scope's route through appRoutes, so the route table stays the one component map. A non-canonical path renders the *all catch-all, which redirects home as before.
• store.ts: adds focusedView. view, selectedChannelId, selectedTopicId and activeRepoId now derive from it. The router hash goes to the focused view, and a redirect the view makes itself is pushed back to the URL. applyRoute and the apply*Route functions are gone. Issue selection stays window-wide (§A1) and is re-anchored on each agent entry, as before.
• App.tsx renders <ViewHost scope={store.focusedView()} /> in <main>. AgentView, LogPanel, TopicView and ChannelView (when it has no channel prop) read useView().

One test deleted: store.test.ts › openAgent › a standalone channel selection does not move the workspace channel. It held a standalone channel and an agent workspace in one view at once. With a view owning its route, openChannel moves the focused view off the agent, so that state can't exist. Per-view independence is now covered by ViewHost.test.tsx. The mirror case (re-opening the agent-view agent shows its home DM…) still passes unchanged.

Tests

• Red-green ViewHost.test.tsx: two ViewHosts on /agent/acc-compass-ui and /agent/acc-compass-server. Before the change: 0 pass / 2 fail (each view renders its own agent: expected compass-ui, got undefined; opening a terminal tab in one view leaves the other unchanged: expected 1, got 0). After: 2 pass.
• moon run compass-ui:test: 1213 pass, 0 fail. typecheck and stylelint pass. biome reports 0 errors on the changed files; its 3 complexity warnings in store.ts already exist on the parent.
• Playwright visual-smoke and dev-boot: 13 passed, with no baseline changes.

Spec-impact: none.
Ledger-impact: none

Co-authored-by: Matt Wilkinson matt@rigel.build

[linear-code]

RIG-4442

[github-actions]

Compass engineering docs preview: https://compass-ui-rig-4442-surfaces.compass-eng-docs.pages.dev

Deployed from compass-ui/rig-4442-surfaces-use-view at baa006f.

[trunk-io]

This pull request is queued for merge as part of 1798, which will merge 1652, 1675, 1743, 1755, 1770, 1780, 1787, 1798.

[trunk-io]

This pull request was merged into main as part of stacked PR 1798.