bf4a55f2f2
Bring buildErrandPrompt() and buildErrandRevisionPrompt() in line with the codebase pattern used by execute, plan, detail, etc. Import shared DEVIATION_RULES and GIT_WORKFLOW constants. Add session_startup (pwd, git status, CLAUDE.md, expected-pwd.txt), execution_rules, and anti_patterns sections. Keep signal format inline since errands use result.message instead of the standard questions format.