ADR-0021: Task pull requests through the gh CLI

Status: Accepted, 2026-10-05. Spec: 020-pr-cycle.

Context

ADR-0020 deferred the PR/CI cycle: a finished task could only merge locally. Opening a PR and reading CI and review state needs GitHub credentials. The server runs as the user and already shells out to git.

Decision

• The server runs the user's gh CLI (gh pr create, gh pr view --json) and git push -u origin <branch> from the task worktree. signaltty stores no token and adds no HTTP client: auth, hosts and enterprise config are whatever gh auth already has.
• Each call runs off the async runtime with a 60 s deadline. Failure records nothing; gh's stderr comes back in details.stderr.
• A PR is not a disposition. Task.pr sits beside disposition, so the existing finish paths keep their meaning: discard cleans up after the PR lands, and a local merge is refused while a PR is open.
• State is read on demand (task.pr_refresh, and when the board opens), not polled. Polling, CI auto-paste and gh pr merge are additive later.

Consequences

gh becomes an optional runtime dependency: only task.pr_* need it, and its absence is a clean SPAWN_FAILED. Tests use a fake gh script on the server's PATH and a local bare repository as origin.