Plural Pull Request Data Model And API

Summary

Make runtime pull request records plural now: a run has pull_requests: [], not pull_request: null.
Absorb the breaking API change now and remove singular runtime PR API fields/routes.
Keep end-user surfaces simple: CLI and web continue showing/acting on the first PR only, with no multi-PR UI or chooser.
Do not change run configuration settings like [run.pull_request]; that still controls whether a workflow auto-opens a PR.

Update OpenAPI runtime schemas so RunProjection, RunSummary, and board RunListItem expose pull_requests: PullRequestRecord[] or RunPullRequest[]; remove their singular pull_request fields.
Replace singular PR routes with plural routes:
- POST /api/v1/runs/{id}/pull_requests creates and links a new GitHub PR.
- GET /api/v1/runs/{id}/pull_requests lists stored PR records.
- POST /api/v1/runs/{id}/pull_requests/link links an existing PR record without touching GitHub.
- POST /api/v1/runs/{id}/pull_requests/unlink removes the association without closing the PR.
- GET/POST /api/v1/runs/{id}/pull_requests/{owner}/{repo}/{number} style targeted detail/merge/close routes operate on a specific stored PR.
Add schemas for PullRequestKey, LinkRunPullRequestRequest, and UnlinkRunPullRequestRequest. Link/create requests may include primary: boolean; the first PR is primary by default, and primary means "move to index 0."
Replace projection events with action-oriented events:
- pull_request.linked with { pull_request, source, primary }
- pull_request.unlinked with { pull_request, reason? }
- pull_request.create_failed with { error }
Remove pull_request.created / pull_request.failed from canonical event handling unless implementation needs temporary test fixture cleanup during the refactor.

In fabro-types and the store reducer, replace RunProjection.pull_request: Option<PullRequestRecord> and RunSummary.pull_request with pull_requests: Vec<PullRequestRecord>.
Add helper behavior around PR keys: upsert linked records by (owner, repo, number), move primary records to index 0, and remove matching records on unlink.
Update workflow PR creation to emit pull_request.linked with source: "created" after GitHub succeeds, and pull_request.create_failed on failure.
Update server PR handlers to append link/unlink events rather than mutating state directly; remove the old "any PR exists" conflict because multiple PRs are now valid.
Update CLI fabro pr create/view/merge/close to keep singular UX:
- create calls the plural create route and prints the returned URL.
- view/merge/close list PRs, select pull_requests[0], then call the targeted plural route.
- If the list is empty, preserve the current "No pull request found" style error.
Update web data mapping in apps/fabro-web/app/data/runs.ts to read the first item from pull_requests; do not add any new UI controls or multiple-PR display.
Regenerate Rust and TypeScript API clients after editing docs/public/api-reference/fabro-api.yaml.

Add/update store reducer tests for link, idempotent upsert, primary reordering, unlink, and summary projection.
Add/update event conversion/name/pretty-output tests for pull_request.linked, pull_request.unlinked, and pull_request.create_failed.
Update server tests for plural route paths, list/link/unlink, targeted detail/merge/close, and create persisting into pull_requests.
Update CLI PR command tests to mock plural endpoints while keeping user-facing output singular.
Update web data mapper tests to prove only pull_requests[0] is surfaced in current cards.
Run:
- cargo build -p fabro-api
- cd lib/packages/fabro-api-client && bun run generate
- cargo nextest run -p fabro-store -p fabro-workflow -p fabro-server -p fabro-cli
- cd apps/fabro-web && bun test && bun run typecheck
- cargo +nightly-2026-04-14 fmt --check --all
- cargo +nightly-2026-04-14 clippy --workspace --all-targets -- -D warnings

Breaking runtime API changes are acceptable; no singular runtime PR fields/routes are kept for compatibility.
No production data migration is needed. Existing local/dev event logs using old PR events may be treated as stale.
pull_requests[0] is the primary PR for current CLI/web behavior.
Link/unlink only changes Fabro's association to a PR; merge/close are the only operations that mutate GitHub state.