On a client's first screen each app row says "Working on: …" or "Last change: …, 3 days ago" #4008

Second half of #3344. It renders what clientDoings returns.

Done:

• [ ] Each app row in src/web/studio/client_home.ts gets one more plain sentence under agentsLine: "Working on: <title>" or "Last change: <title>, <relative time>" (reuse ago from src/feed/rank.ts). A quiet app gets no extra line.
• [ ] Titles are escaped by the html template and cut to 80 characters at a word boundary with "…".
• [ ] No developer words (pull, merge, branch, commit, an issue number). The word list test/client-home.test.ts holds still passes.
• [ ] The read is its own doing phase in Server-Timing, and the apps phase is unchanged.
• [ ] Tests: the working, landed and quiet rows, escaping, the 80-character cut, and the doing phase.
• [ ] After pushing, fetch the preview's / and a client home. Both answer 200. The two earlier attempts (#3353, #4005) were closed because the preview's deploy failed, so say in the pull that the preview booted.

Files: src/web/studio/client_home.ts, test/client-home.test.ts.