fix(components): mark unread sub-session tabs on desktop

[lodystage]

Related issue

Problem / pressure

A child session tab carried no unread indicator on desktop. Sub-sessions get no
sidebar row of their own, so the tab is the only place their new output can
surface — a subagent that finished while the user was reading the parent thread
stayed completely silent until they happened to click its tab.

While fixing that, the same slot turned out to have a dead branch: isWaiting
was tested after isWorking, but a permission request is itself live presence
(liveStatus != null), so a tab blocked on approval always rendered the busy
spinner and its own marker never reached the screen. "This tab needs you" was
indistinguishable from "this tab is busy".

Summary

The tab's leading icon becomes ONE priority-ordered status slot,
waiting > working > unread > agent icon — the same ladder the desktop sidebar
row (sidebar-row-shared.tsx) and the mobile tab sheet already use.

• Unread reads through the new sessionHasUnreadMessages
  (lib/session-read-receipt.ts), which shares its comparison with
  shouldMarkSessionRead, so announcing unread cannot drift from deciding
  the read receipt.
• Unread is suppressed on the ACTIVE tab: that surface is the one clearing
  unread, so a dot there would only flash between the click and the receipt
  landing.
• Waiting renders the sidebar's Hand instead of the old amber dot.
  --primary and --status-warning are both amber in the shipped themes, so an
  amber waiting dot beside a primary unread dot read as the same marker (caught
  from the screenshots, not from the code).
• The slot is a fixed 12px box, so switching states never shifts a tab label.

The invariant is recorded in components/sessions/AGENTS.md.

Before / after

Story Sessions/SessionTabBar › Unread sub-session tabs, six tabs: active
parent · unread child · never-read child · working child · child awaiting
permission · caught-up child.

Before	After
No unread marker anywhere — the two unread children are indistinguishable from the caught-up one (all three show the plain agent icon).	Both unread children carry the bg-primary dot (blue in light, amber in dark).
The child awaiting permission renders a spinner, identical to the merely-busy child (two spinners in the row).	It renders the amber Hand; exactly one spinner remains.
—	The active parent stays unmarked.

DOM counts from the Playwright capture, both themes:
before = 0 unread dots / 2 spinners / 0 hands,
after = 2 unread dots / 1 spinner / 1 hand.

Test plan

• New packages/components/tests/session-tab-bar-status-slot.test.tsx (7 cases):
  unread child, never-read child, caught-up child, unread parent while a child
  tab is active, active tab not marked, spinner outranks unread, hand outranks
  spinner. Verified they are not vacuous — run against the pre-fix
  session-tab-bar.tsx, 4 of the 7 fail.
• New story Sessions/SessionTabBar › Unread sub-session tabs covering the
  whole ladder in one row; screenshots above were captured from it in both
  themes by swapping the component back to HEAD for the "before" pass.
• pnpm typecheck (components), oxlint, pnpm lint:i18n: clean.
• Full @lody/components unit suite: 422 files / 3044 tests passing.
• Not run: the Playwright e2e suite (no story render-budget test references
  SessionTabBar), and the other workspace packages, which this change does not
  touch.

Note for anyone reproducing locally: this environment exports
NODE_ENV=production, under which React 19 does not export act and every
pre-existing jsdom test in the package fails with
(0 , act) is not a function. Unrelated to this change — the runs above used
NODE_ENV=development.