feat(sidebar): make the work card's brief the plan's active step

[ElbertePlinio]

#361 PR 2 — the issue's primary gap.

The renderer existed and cardBrief was wired to it, but taskBriefText has no writer outside fixtures. workspace.ts always creates chats with taskBriefText: null; the only non-null values are VRT mocks. So the middle of every real card — the part that says what the agent is doing — was empty, and the card collapsed to mark · project · time / title · WORKING / $1.88.

The brief is the plan's active step

Per the decision recorded on the issue. This needs no new plumbing: PlanItemStatus already carries inProgress, and latestPlanForChat is the same accessor cardPlanProgress already reads for plan M/N.

That is the point. The card says step 2 of 5 in its footer and what step 2 is on this line, from one source — they cannot drift apart. taskBriefText becomes the fallback slot rather than the only source, lighting up when #210's one-liner producer lands.

Verified in the rendered app

{ title: "Sidebar waiting state", brief: "Wire the swarm survey lanes into the footer", plan: "plan 2/5" }
{ title: "PR monitoring agent flow", brief: "Wants to run: gh pr checks --watch",        plan: null      }

The card with a plan shows its actual in-progress step next to the matching tally; the card without one falls through to its one-liner. Not just asserted — read off the running app.

Sub-decisions resolved

• Truncation → capped in the store at 120 chars, newlines flattened. The line is single-line-ellipsis in CSS anyway, so anything past that is invisible; a persisted value has no reason to be longer than what can ever be read.
• Precedence → plan step beats a cached one-liner, made explicit in the resolver so it cannot drift.
• All-complete plan (M === N, no inProgress) → falls through to the one-liner, then to nothing. Showing the last completed step would claim work that is already finished.

An inProgress step with only whitespace is ignored rather than rendered as a blank line, matching the locked no-placeholder rule.

Tests

8 new, verified load-bearing (removing the plan-step path fails 3):

• in-progress step wins; beats a cached one-liner; all-complete falls through; finished-with-no-fallback shows nothing; no plan at all falls through; whitespace-only step ignored; long step flattened and capped; still works with no plan accessor.

Validation

• bun run test:unit — 1619 passed
• bunx tsc --noEmit clean; bun run lint clean
• Rendered check above, on an isolated port

Remaining on #361

PR 3 — the model-written one-liner producer — is not here. The issue says to sequence it with #210 so one producer serves both, and building a second one here would be exactly the duplication that decision exists to prevent. The fallback slot is in place and tested for when it lands.

Refs #361