feat(commands): add /handoff builtin command with programmatic context synthesis

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

src/features/builtin-commands/commands.ts

@@ -5,6 +5,7 @@ import { RALPH_LOOP_TEMPLATE, CANCEL_RALPH_TEMPLATE } from "./templates/ralph-lo
 import { STOP_CONTINUATION_TEMPLATE } from "./templates/stop-continuation"
 import { REFACTOR_TEMPLATE } from "./templates/refactor"
 import { START_WORK_TEMPLATE } from "./templates/start-work"
+import { HANDOFF_TEMPLATE } from "./templates/handoff"
 
 const BUILTIN_COMMAND_DEFINITIONS: Record<BuiltinCommandName, Omit<CommandDefinition, "name">> = {
   "init-deep": {
@@ -77,6 +78,22 @@ $ARGUMENTS
 ${STOP_CONTINUATION_TEMPLATE}
 </command-instruction>`,
   },
+  handoff: {
+    description: "(builtin) Create a detailed context summary for continuing work in a new session",
+    template: `<command-instruction>
+${HANDOFF_TEMPLATE}
+</command-instruction>
+
+<session-context>
+Session ID: $SESSION_ID
+Timestamp: $TIMESTAMP
+</session-context>
+
+<user-request>
+$ARGUMENTS
+</user-request>`,
+    argumentHint: "[goal]",
+  },
 }
 
 export function loadBuiltinCommands(