base_branch support for stacked PRs

[chanakyav]

Parent Epic

Part of #52 — Sequential stacked PR chains

Summary

Add base_branch support so PRs can target a parent task's branch instead of the repo's default branch. This enables true stacked PRs where PR #2 targets PR #1's branch.

Context

Today, all PRs implicitly target main (or whatever the default branch is). Prompts tell the agent gh pr create --draft without specifying --base. For stacked workstreams, PR #2 needs to target PR #1's branch so it only contains its own diff.

Changes

persistence.py

• base_branch column added as part of schema v9 (done in Schema: parent_task_id, epic_id, base_branch, upstream_sha columns #55)
• Add to _TASK_COLUMNS frozenset (done in Schema: parent_task_id, epic_id, base_branch, upstream_sha columns #55)

prompts.py

• All prompts that mention gh pr create add --base <base_branch> when base_branch is set
• Self-review diffs against base_branch: git diff <base_branch> instead of hardcoded main
• Fix prompt to diff against base_branch too

github_api.py

• Add get_default_branch() — gh repo view --json defaultBranchRef --jq .defaultBranchRef.name, cached
• Update find_pr_for_branch() to optionally verify the base branch matches

orchestrator.py

• Orchestrator reads base_branch from task record
• Passes base_branch to prompt generators
• Falls back to get_default_branch() when base_branch is NULL
• Fix _do_update_description() to use base_branch instead of hardcoded main for git diff

cli.py

• cmd_start accepts --base flag for manual stacked PR support
• Stores base_branch in task record

Depends On

• Schema: parent_task_id, epic_id, base_branch, upstream_sha columns #55 (schema — provides the base_branch column)

Testing

• PR created with --base <parent_branch> when base_branch is set
• PR created without --base when base_branch is NULL (targets default branch)
• get_default_branch() caches correctly
• Prompts include correct diff target
• _do_update_description() uses base_branch for git diff