github: handle duplicate PR creation when agent creates PR

[elithrar]

Fixes a false-negative workflow failure when the OpenCode agent creates a PR via gh pr create during its run - the post-run infrastructure then attempts to create a duplicate PR for the same branch, causing the workflow to fail with "A pull request already exists".

• Check for existing open PRs with the same head→base combination before calling pulls.create (unique for any open PR)
• Return the existing PR number if found, avoiding the duplicate PR error
• Attempts to create the PR (no worse than existing behavior) if it cannot find a pull
• a simple withRetry to retry the PR check and PR create API calls (net improvement)

Ref: elithrar/dotfiles#19

[elithrar]

Example of the bug:

• the agent uses the gh CLI to create a PR (not expected by the harness)
• the harness sees a dirty branch and attempts to create a PR (it's normal behvavior)
• PR already exists for that base:head combo and "fails", but "a" PR was created (not great, Bob)

[elithrar]

logs - note the agent called gh pr and the harness failing after:

|  Bash     {"command":"gh pr create --title \"Add worktree plugin for concurrent branch development\" --body \"$(cat <<'EOF'\n## Summary\n\n- Adds a worktree plugin (`.opencode/plugin/worktree.ts`) with a `use-worktree` tool for concurrent branch development\n- Implements session-scoped worktrees at `/tmp/opencode-worktree-{sessionID}/` to avoid conflicts between sessions\n- Supports user-defined merge strategies (`ours`, `theirs`, `manual`) with conflict resolution guidance\n\n## Key Features\n\n### Agent Hooks\nThe tool exposes clear actions for the agent to use:\n- `create`: Create a new worktree for a branch (with optional `createBranch` flag)\n- `list`: List all worktrees in the current session\n- `remove`: Remove a specific worktree\n- `merge`: Merge worktree changes back to a target branch\n- `status`: Get the status of a worktree (changes, commits ahead/behind)\n- `cleanup`: Remove all session worktrees\n\n### Merge Strategies\n- **ours**: On conflict, keep changes from the target branch\n- **theirs**: On conflict, keep changes from the worktree branch\n- **manual**: Stop on conflict and return diff for user decision\n\n### Crash-Proof Design\n- All exceptions are caught and emitted as error messages\n- Plugin never throws unhandled exceptions that could crash OpenCode\n- `safeExec` wrapper handles all shell command failures gracefully\n- Event handlers wrapped in try/catch\n\n### Automatic Cleanup\n- Subscribes to `session.deleted` and `session.error` events\n- Automatically removes all session worktrees when session ends\n- Prunes dangling worktree references\n\n## Files Changed\n- `.opencode/plugin/worktree.ts` - The worktree plugin implementation\n- `.opencode/package.json` - Plugin dependencies (`@opencode-ai/plugin`)\n- `.gitignore` - Exclude plugin `node_modules/` and `bun.lock`\n\nCloses #17\nEOF\n)\"","description":"Create PR for worktree plugin"}


Created PR #18: https://github.com/elithrar/dotfiles/pull/18

## Summary

<snip>

Checking if branch is dirty...
368e353b5d39d321bcad48c8ee79662b1cd17dbe
Sending message to opencode...
Add worktree plugin with tool (PR #18)

Pushing to new branch...
branch 'opencode/issue17-20260103124524' set up to track 'origin/opencode/issue17-20260103124524'.
Everything up-to-date
Creating pull request...
POST /repos/elithrar/dotfiles/pulls - 422 with id 2028:22DF8B:BF2795A:358685D8:6959106A in 331ms
118 |       request: requestOptions
119 |     });
120 |   }
121 |   if (status >= 400) {
122 |     octokitResponse.data = await getResponseData(fetchResponse);
123 |     throw new RequestError(toErrorMessage(octokitResponse.data), status, {
                ^
HttpError: Validation Failed: {"resource":"PullRequest","code":"custom","message":"A pull request already exists for elithrar:opencode/issue17-20260103124524."} - https://docs.github.com/rest/pulls/pulls#create-a-pull-request
   status: 422,
  request: {
  method: "POST",
  url: "https://api.github.com/repos/elithrar/dotfiles/pulls",
  headers: [Object ...],
  body: [Object ...],
  request: [Object ...],
},
 response: {
  url: "https://api.github.com/repos/elithrar/dotfiles/pulls",
  status: 422,
  headers: [Object ...],
  data: [Object ...],
},

[elithrar]

test failures are from the worktree changes and are failing on the current dev tip (f6cc84747)

[rekram1-node]

/review

[github-actions]

lgtm