fix(github): action branch detection and 422 handling

[elithrar]

When the agent creates its own branch, commits, and pushes during chat(), the infrastructure's post-chat flow pushes a stale branch ref (still pointing at main's HEAD) and tries to create a PR — which GitHub rejects with 422 "No commits between main and ".

The sequence that triggers this:

• automation creates opencode/issue127-... from main HEAD, captures HEAD SHA
• Agent runs, creates its OWN branch (fix/...), commits, pushes, and creates a PR via gh pr create
• branchIsDirty sees HEAD changed → dirty=true, but we're now on the agent's branch, not opencode/issue127-...
• pushToNewBranch pushes opencode/issue127-... which still points to main → 422

Changes:

• branchIsDirty now checks whether the agent switched branches during chat via git rev-parse --abbrev-ref HEAD. Returns switched: true when the current branch doesn't match the expected one.
• All three event paths (repo events, PR events, issue events) handle switched by skipping infrastructure push/PR — the agent already handled it.
• createPR verifies commits exist between base and head via git rev-list --count before calling the GitHub API, with a fetch fallback for shallow clones.
• createPR catches 422 "No commits between" and returns null instead of throwing — callers fall back to commenting.

example failure

Pushing to new branch...
remote: 
remote: Create a pull request for 'opencode/issue127-20260219011637' on GitHub by visiting:        
remote:      https://github.com/cloudflare/somerepo/pull/new/opencode/issue127-20260219011637        
remote: 
To https://github.com/cloudflare/somerepo
 * [new branch]      opencode/issue127-20260219011637 -> opencode/issue127-20260219011637
branch 'opencode/issue127-20260219011637' set up to track 'origin/opencode/issue127-20260219011637'.
Creating pull request...
POST /repos/cloudflare/somerepo/pulls - 422 with id 5818:30FF9E:37E4EA:390DFD:6996671E in 421ms
Retrying after 5000ms...
POST /repos/cloudflare/somerepo/pulls - 422 with id 5818:30FF9E:37FE3A:3927D1:6996671E in 426ms
Validation Failed: {"resource":"PullRequest","code":"custom","message":"No commits between main and opencode/issue127-20260219011637"} - https://docs.github.com/rest/pulls/pulls#create-a-pull-request
Creating comment...
Removing reaction...
Error: Validation Failed: {"resource":"PullRequest","code":"custom","message":"No commits between main and opencode/issue127-20260219011637"} - https://docs.github.com/rest/pulls/pulls#create-a-pull-request
Error: Process completed with exit code 1