Agent cards as workspaces: click-to-filter running tabs, per-card ＋, status badges

[astetic-dev]

Problem

The tab bar is the bottleneck for heavy multi-agent use. Two real-world patterns
both overflow it:

1. Parallel work on one process — several agents on the same folder (one
   fixes, one tests, one reviews). Tabs get near-identical names.
2. Many processes at once — one or two sessions each for many agents. With
   10+ tabs the bar becomes unreadable and scrolls horizontally.

The sidebar cards are the natural place to organize this: a card should act as
a workspace over its running sessions, not just a launch button.

Behaviour (decided)

• Card with no running sessions — a click is a start-up: it opens the
  launch form for that agent (current behaviour).
• Card with running sessions — a small ＋ appears on the card element:
  • the ＋ opens the new-agent/launch form (prefilled with the card's
    defaults) to start another session for this card;
  • a plain click on the card toggles a filter: the tab bar shows only this
    card's sessions;
  • clicking the card again clears the filter.
• While a filter is active it must be clearly visible in the UI (e.g. the
  card highlighted + a filter indicator at the tab bar).
• Ad-hoc sessions (started via browse-folder): no separate entry in the
  left menu. They are simply visible whenever the filter is inactive, and
  hidden by any card filter.
• Status badge per card: running-session count + aggregated state — steady
  when working, flash in the card colour when a session awaits input — so a
  background agent's "ready" is never missed while you're filtered onto another
  workspace. This makes the sidebar double as a status dashboard.
• Ctrl+Tab cycles within the active filter (all tabs when no filter).

Technical notes

• Sessions currently do not record which card they were launched from
  (spawnTerminal gets path/title/accent/mode/agent/model only). Add a
  projectId to the session object and to PersistedSession, so grouping
  survives restarts. Ad-hoc sessions have projectId = "".
• Filtering is a render concern in renderTabs; the ready/flash detection is
  unchanged — the card badge mirrors the same state.
• Relation to the backlog: this supersedes the ideas behind Sidebar: filter input for the project list #42 (filter
  input) and Sidebar: optional collapsible project groups #43 (collapsible groups) — the cards themselves become the
  filter. Decide on closing those when this lands.