* fix: mark task as failed when PR creation fails
Previously, when PR creation failed (e.g. invalid base branch),
the task was still marked as 'completed' even though the PR was
not created. This fix ensures:
- postExecutionFlow returns prFailed/prError on failure
- executeAndCompleteTask marks the task as failed when PR fails
- selectAndExecuteTask runs postExecution before persisting result
The pipeline path (executePipeline) already handled this correctly
via EXIT_PR_CREATION_FAILED.
* fix: use detectDefaultBranch instead of getCurrentBranch for PR base
Previously, baseBranch for PR creation was set to HEAD's current branch
via getCurrentBranch(). When the user was on a feature branch like
'codex/pr-16-review', PRs were created with --base codex/pr-16-review,
which fails because it doesn't exist on the remote.
Now uses detectDefaultBranch() (via git symbolic-ref refs/remotes/origin/HEAD)
to always use the actual default branch (main/master) as the PR base.
Affected paths:
- resolveTask.ts (takt run)
- selectAndExecute.ts (interactive mode)
- pipeline/execute.ts (takt pipeline)
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