feat(issue-desk): reuse existing PR for an issue instead of opening duplicates

[larryro]

Problem

The same issue can spawn several duplicate PRs (e.g. #2095 → #2099 / #2118 / #2120). Root cause: the implementer only checks whether the tale/<task._id> branch has a PR, never whether the issue is already being fixed by some other PR — so every desk run opens a fresh competing PR.

Changes (config/instructions only, no code)

builtin-configs/apps/issue-desk/:

implement step — dedupe before branching:

• Discover open PRs resolving the issue — GitHub closing-reference link (GraphQL closedByPullRequestsReferences) + title/body fallback (#num) + tale/* branches.
• Decide & set $BRANCH: our branch already has a PR → normal rework; exactly one other PR → adopt it and push onto its branch (no new PR); several → keep the furthest-along as canonical and gh pr close the redundant duplicates (with a "Superseded by #N" comment; conservative guardrails: never delete a human's branch, keep + flag any PR carrying unique work).
• All downstream git/gh calls use $BRANCH; new PRs include Closes #num so later runs can resolve them by link.

review step:

• Resolves the same canonical PR via the issue→PR link and checks out its head branch (was a hardcoded tale/<task._id> checkout). The PR's headRefName is the branch the implementer pushed to, so the reviewer always reviews the implementer's work.

Bumped 1.2.0 → 1.3.0; updated the workflow / app / implementer-agent descriptions.

Known limitations

• Cross-step code is carried by the remote git branch + PR (sandboxes don't share state); both steps use the same link query so they converge on the same PR.
• Does not handle two desks operating on the same task concurrently — that needs dedup at task-creation time (platform code), out of scope for this app.