feat: edit the description and pick reviewers from Overview

[omartelo]

What

The Overview tab was a read-only rendering of the pull request's body. Two ordinary things still had to be done on github.com — trimming a description an agent drafted, and choosing who reviews — which is the last gap in a screen that already merges, comments and files a review.

Overview now carries:

• Opened by — the author, which the detail never showed.
• Reviewers — who was asked, who approved, who requested changes, with Request review beside it.
• Edit on the description, in place, saved through gh pr edit --body.

How

• internal/project/prreviewers.go: AssignableReviewers (GraphQL assignableUsers — what gh's own prompt uses, visible to a read-only account), RequestReview (gh pr edit --add/--remove-reviewer), and toReviewers, which merges gh's two halves of the roster.
• internal/project/pr.go: EditPullRequestBody; prViewFields gains author, reviewRequests, latestReviews.
• frontend/src/components/pulls/PullsOverview.tsx: the tab, lifted out of PullRequestView.
• frontend/src/lib/pulls/reviewers.ts: who the picker may offer, and who it ticks.

Decisions worth knowing

• The roster is one list, not two. reviewRequests is who owes an answer, latestReviews is who gave one, and GitHub moves a person between them — a submitted review clears the request, asking again re-files it. Either half alone reads as a lie the moment the first review lands.
• A reviewer who answered comes back unticked. Their request is gone; ticking them again is GitHub's re-request.
• The author is filtered out of the picker — GitHub refuses a review request addressed to them, and an item that can only fail is not an item.
• A merged or closed PR keeps the roster and loses the picker; GitHub would refuse the request anyway.
• A team shows on the roster but not in the picker: gh addresses one as org/slug, which is not the slug the roster carries.
• Ceiling: the picker reads the first 100 assignable users, unpaged. A larger organisation keeps a tail only github.com can reach.

Test plan

• gofmt -l ., go vet ./..., go test ./... (809)
• pnpm check, tsc --noEmit, vitest run (641), vite build
• New: internal/project/prreviewers_test.go (roster merge incl. re-request, teams, deleted accounts; gh args for add/remove/edit) and frontend/src/lib/pulls/reviewers.test.ts
• Rendered in a headless Chromium over CDP (the gate cannot render): Overview tab, the description editor, and the reviewer menu with its items
• The description editor sizes itself to the body between 40vh and 70vh (field-sizing-content), verified in the built CSS and reviewed running under task dev
• Requesting and withdrawing a review against a live open PR — not exercised: this repository lists one assignable user, its own author, whom GitHub refuses as a reviewer