Chief of Staff: show a light time on each message #420

User story

As a person reading the Chief of Staff thread, I want to see when each message was posted, so that I can tell how fresh a reply or request is.

Acceptance criteria

• Each person message shows the time it was sent.
• Each Chief of Staff post shows the time it was posted.
• The time uses light, italic, secondary text that does not compete with the message.
• A message from today shows the time only; an older message also shows the date.
• The time uses the browser's locale and time zone.
• The time is hidden on phone-width layouts.
• Screen readers read the full date and time.

What I'd notice

A small italic time under each message in the Chief of Staff thread on desktop.

Tasks

• Render from the post's existing creation time
• Phone breakpoint hides it
• Playwright check on desktop and phone widths
• docs/FEATURES.md row

Contract

Ownership: public/chief-of-staff-page.js, public/apple/chief-of-staff.css, docs/FEATURES.md (new row G6.82), docs/design-chief-of-staff.md (thread section), test/e2e/g6-cos.spec.js, design/homework/mockups/16-cos-message-time-built*.png.

Behaviour (client only; every post already carries at, an ISO time eve sets in _addPost; no server change):

• Every rendered post (person and Chief of Staff, every kind) ends with one <time class="cos-time" datetime="<post.at>">. A post whose at does not parse renders no time.
• Visible text: Intl.DateTimeFormat(undefined, { timeStyle: 'short' }) when at is on the browser's local today, else { dateStyle: 'medium', timeStyle: 'short' }. That text is aria-hidden; a visually hidden span (.cos-sr) carries { dateStyle: 'full', timeStyle: 'short' } for screen readers.
• Person posts: the bubble keeps exactly the person's text (existing exact-text locators stay valid); the time sits under the bubble, right-aligned, outside it. Every existing testid stays.
• Style: font-style: italic; font-weight: 300; color: var(--text-secondary); footnote size. Hidden (display: none) under the phone breakpoint (max-width: 599.98px).
• Alert and finished posts drop the clock from their meta line (the project stays; an empty meta line is not rendered), so each post shows its time once.

Waits: the spec waits on the post in the feed, then reads its time; no durations.

Decisions made for the owner

• One time per post: alerts and finished posts move their clock from the top meta line to the light time under the post. On phones they then show no clock, like every other post; the card's "Waiting on you · 5 min" age stays.
• "Today" is decided when the post renders; a page left open past midnight shows yesterday's posts time-only until the next render.