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.
bf4a55f2f2