Board: implement the work-first Now, Timeline, Releases, and Health views

[jeffhuber]

Part of #945 and roadmap #900.

Problem

After truth semantics are corrected, Board still needs a compact work-first interaction model that makes current work, evidence, freshness, and next action understandable at desktop and phone widths without a long wall of cards.

Scope

Implement the approved local Board structure: persistent compact chrome; Now, Timeline, Releases, and Health views; work-first rows; adjacent desktop or inline mobile selected-work detail; participant strip; and meaningful recent changes. Render the accepted B0 fixtures before automatic producers land. Keep completed campaigns collapsed under Releases and connection/version/process diagnostics under Health.

Primary actions remain read-only links such as Open PR, View review, Inspect connection, and View evidence. Do not add merge, requeue, force-lease, cancel, retry, restart, cloud-schema, or Slack-specific controls.

Dependencies

• Board: freeze the provider-neutral observation contract and truth fixtures #946 — observation contract and truth fixtures.
• Board: correct existing-data truth, hierarchy, and attention semantics #947 — existing-data truth, hierarchy, and attention semantics.

Acceptance criteria

• At desktop and phone widths, an operator can find active work, source freshness, one meaningful next step, and responsible role without traversing release history.
• Work rows show safe reference/title, stage, assignments when known, last meaningful update, next action, and responsible role; selecting one exposes independent builder/review/CI/gate/merge/human-policy evidence.
• Review requested, review observed running, stale review, changes requested, implementation complete, ready for human review, ready to merge, and merged remain distinct.
• Tab/row selection is keyboard accessible, focus and expanded state are visible, mobile detail follows the row, and meaningful changes are announced without announcing every poll.
• Selection remains stable during refresh; unchanged polling snapshots collapse from Timeline.
• No percentage or ETA is invented; color always has text/icon and unknown is neutral.
• No-session, stale-source, source-unavailable, idle-with-complete-coverage, and partial-coverage fixtures produce honest summaries.
• Rendering remains local and adds no cloud fields or private data.

Initial ownership

Presentation track, sequential after B1. Planned builder: Code Mower Claude.

Code Mower delivery

Produce one independently reviewable PR for this issue. Record the named Code Mower builder, keep one writer on the branch, obtain an independent audit against the exact current head, resolve every P0/P1/P2 finding, and pass focused tests, desktop/mobile/accessibility checks, relevant full tests, privacy checks, normal CI, and code-mower/gate. Upload only allowlisted metadata. No umbrella PR.