feat: activate coordinator mode in open build (#647)

* feat: activate coordinator mode in open build

Enable the COORDINATOR_MODE feature flag and create the missing
src/coordinator/workerAgent.ts module that provides worker agent
definitions for the coordinator.

Coordinator mode is a multi-agent system where a coordinator agent
orchestrates independent workers via AgentTool, SendMessageTool,
and TaskStopTool. The implementation was already 99% complete
(19KB coordinatorMode.ts, 26 gate sites across 15 files) — only
the workerAgent module was missing from the source snapshot.

Workers get the standard built-in agents (general-purpose, explore,
plan). The coordinator system prompt (252 lines) handles all
orchestration logic.

Activate at runtime: CLAUDE_CODE_COORDINATOR_MODE=1
Optional scratchpad: set {"tengu_scratch": true} in
~/.claude/feature-flags.json (#639)

* fix: add worker agent type for coordinator mode

The coordinator system prompt instructs the model to spawn workers with
subagent_type: "worker", but no agent had agentType === 'worker'.
This caused AgentTool to throw "Agent type 'worker' not found" on
every coordinator spawn attempt.

Add a WORKER_AGENT definition that spreads GENERAL_PURPOSE_AGENT with
agentType: 'worker'. Also use the narrower BuiltInAgentDefinition type.

* feat: activate built-in explore and plan agents in open build

Enable BUILTIN_EXPLORE_PLAN_AGENTS so Explore (fast, haiku, read-only)
and Plan (architect, read-only) agents are available to all users in
both normal and coordinator modes.

This resolves the inconsistency flagged in code review: coordinator
workers had access to Explore/Plan agents while normal sessions did not.

The GrowthBook A/B test gate (tengu_amber_stoat) defaults to true via
the no-telemetry stub. Users can disable via feature-flags.json (#639).

scripts/build.ts

@@ -29,7 +29,8 @@ const featureFlags: Record<string, boolean> = {
   ABLATION_BASELINE: false,
   DUMP_SYSTEM_PROMPT: false,
   CACHED_MICROCOMPACT: false,
-  COORDINATOR_MODE: false,
+  COORDINATOR_MODE: true,
+  BUILTIN_EXPLORE_PLAN_AGENTS: true,
   CONTEXT_COLLAPSE: false,
   COMMIT_ATTRIBUTION: false,
   TEAMMEM: false,