A pull request's title, description and work item can be edited after it opens

After a pull request opens, its title, description and work item can't be changed.

Today

• UpdatePullArgs (crates/contracts/src/work.rs:2074) covers assignees, reviewers, labels, milestone and base, and nothing else.
• The MCP pull_request update action accepts body and item and silently ignores them. It answers with the pull request unchanged, so a caller thinks the edit worked.
• The CLI has no g1t pr edit.

So Closes #N (read from the description by refs::named) only counts if it was written when the PR opened. On 2026-10-21, PR #338 couldn't be tied to #339, which was filed a minute after it.

Done when

• Updates: title and description can be updated through the RPC, API, MCP (update) and the CLI (g1t pr edit --title/--body/--body-file), plus an edit control on the PR page. The PR's author, or someone with Write, can make the change.
• Links: after an edit, the closing and Part of references are read again. Items it no longer names are unlinked; newly named ones are linked, under the same visibility rules (admits).
• Work item: a PR's own work item can be set or changed (item).
• No silent ignores: MCP and API calls that pass a field the action doesn't take are refused rather than ignored.
• Docs: the pull requests guide says how to edit and how references are re-read.