Port handoff concept from ampcode as a builtin command that extracts
detailed context summary from current session for seamless continuation
in a new session. Enhanced with programmatic context gathering:
- Add HANDOFF_TEMPLATE with phased extraction (gather programmatic
context via session_read/todoread/git, extract context, format, instruct)
- Gather concrete data: session history, todo state, git diff/status
- Include compaction-style sections: USER REQUESTS (AS-IS) verbatim,
EXPLICIT CONSTRAINTS verbatim, plus all original handoff sections
- Register handoff in BuiltinCommandName type and command definitions
- Include session context variables (SESSION_ID, TIMESTAMP, ARGUMENTS)
- Add 14 tests covering registration, template content, programmatic
gathering, compaction-style sections, and emoji-free constraint
a5489718f9