Add an open_pull_request tool that creates a new PR via the GitHub REST
API using the triggering user's OAuth token (resolved by login from the
dashboard store), so the PR creator is the user rather than open-swe[bot].
Falls back to the GitHub App installation token for GitHub-triggered runs,
unmapped users, and bot-token-only deployments.
The user token never enters the sandbox: clone/push/comments still go
through the bot proxy via gh. The agent is steered to use the tool only
for OPENING a new PR; updates (body edits, mark ready) and pasted/existing
PRs continue to use gh pr edit. Existing-PR (422) returns the open PR's
URL so re-runs don't create duplicates.
