Talk opened from a repository, issue or pull starts about that thing: /talk?about= names the subject #10326

Replaces #7640, whose conversation is built on main (/talk, holds, trail, resume; #7647–#7658). What it still lacks is the ask's 'from any page': the header's Talk link (src/web/layout.ts, the g t button and the nav entry) is a bare /talk, so the agent never knows what the owner was looking at.

Done when:

• [ ] On a repository, issue or pull page the header's Talk link is /talk?about=owner/repo, owner/repo#N (issue or pull); on any other page it stays /talk.
• [ ] /talk with a valid about the viewer can read shows one line, 'About owner/repo#N: <title>', above the composer; the first message sent carries that subject to the agent as context, fenced as data like other read text (src/core/fence.ts).
• [ ] about is validated at the boundary: malformed, unknown or unreadable to the viewer is ignored (the page answers 200, names nothing, leaks nothing about a private repository).
• [ ] Tests in test/talk-page.test.ts: the link carries about on an issue page; a private repository's about names nothing for another person; a malformed value is ignored. Run bun scripts/design-lint.ts.

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

Parked by the crew: the check stayed red through 2 automatic tries: the file hold form > over the settings route, a missing and an unreadable repository answer alike (pull #10333 closed; its branch scout/10326-talk-opened-from-a-repository-issue-or-p at 53916a263b09 keeps the work so far). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check stayed red through 2 automatic tries (pull #10376 closed; its branch scout/10326-talk-opened-from-a-repository-issue-or-p at 5a82b0888266 keeps the work so far).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check stayed red through 2 automatic tries: the file hold form > over the settings route, a missing and an unreadable repository answer alike (pull #10333 closed; its branch scout/10326-talk-opened-from-a-repository-issue-or-p at 53916a263b09 keeps the work so far)
2. the check stayed red through 2 automatic tries (pull #10376 closed; its branch scout/10326-talk-opened-from-a-repository-issue-or-p at 5a82b0888266 keeps the work so far)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Split into #10494 (the /talk page reads and validates about), #10495 (the header link carries it, set by the page that rendered the thing, never derived from the URL path) and #10496 (the first message gives the agent the subject, fenced). Why it went red: the link came from the path, so a 404 under a private repository and one under a missing repository rendered different headers, and test/file-hold-form.test.ts ("a missing and an unreadable repository answer alike") caught the leak. #10495 names that test as one that must pass unchanged.