feat: branch selection ability

[andrew-t-james-wc]

Thanks for this project, really enjoying using it. I hit an issue with branch assignment for a recent project. Let me know if this is something you would like to support.

Let users specify which branch AI agents create feature branches from
via --branch / -b on both run and task run commands. Defaults
to the current git branch when omitted.

• Add flag to run and task run commands
• Inject branch instructions into plan and implement prompts
• Include branch in PR metadata block and run history
• Show branch in preflight summary (plain and colored)
• Add DB migration for run_history.branch column
• Add comprehensive test coverage (13 new tests)

My default branch in the repo was set to develop but for some reason it worked from main. This was after one iteration, was unfortunate unusable.

related to #10