The first message sent from /talk?about= gives the agent that subject as fenced context #10496

Third part of #10326.

Done when:

• [x] The composer on /talk?about=… (a valid, readable subject, as resolved by the parser from the first part) posts the subject with the first message; src/actions/talk.ts re-validates it against the sender's read access (never trusts the form) and prepends the subject's kind, name, title and body to the agent's context, fenced as data with src/core/fence.ts like other read text.
• [x] Only the first message of a new conversation carries it; later messages do not repeat it.
• [x] Tests: a test reads the fenced subject back from the agent's input for a readable issue; a forged about naming a private repository the sender cannot read adds nothing.

Files: src/web/pages/talk.ts, src/actions/talk.ts, test/talk-page.test.ts.

Reviewed pull #10662 at its head: ticked The composer on /talk?about=… (a valid, readable subject, as resolved by the parser from the first part) posts the subject with the first message; src/actions/talk.ts re-validates it against the sender's read access (never trusts the form) and prepends the subject's kind, name, title and body to the agent's context, fenced as data with src/core/fence.ts like other read text.; Only the first message of a new conversation carries it; later messages do not repeat it.; Tests: a test reads the fenced subject back from the agent's input for a readable issue; a forged about naming a private repository the sender cannot read adds nothing..