feat: Switch cw ask from polling to SSE via onConversationAnswer subscription

- New onConversationAnswer subscription: listens for conversation:answered
  events matching a specific conversation ID, yields the answer text
- cw ask now subscribes via SSE instead of polling getConversation
- Removed --poll-interval and --timeout flags from cw ask
- Updated prompt to reflect SSE-based cw ask (no polling options)

src/agent/prompts/shared.ts

@@ -60,14 +60,12 @@ Output: \`{ "conversationId": "...", "fromAgentId": "...", "question": "...", "p
 Exits with code 1 on connection error.
 
 **\`cw ask "<question>" --from ${agentId} --agent-id <TARGET_AGENT_ID>\`**
-Ask another agent a question. Blocks until the answer arrives, then prints the answer text to stdout and exits with code 0.
+Ask another agent a question. Blocks via SSE until the answer arrives, then prints the answer text to stdout and exits with code 0.
 Target (exactly one required):
   - \`--agent-id <id>\`: Ask a specific agent directly by agent ID
   - \`--task-id <id>\`: Ask whichever agent is currently running that task
   - \`--phase-id <id>\`: Ask whichever agent is running a task in that phase
-Options:
-  - \`--poll-interval <ms>\`: Answer polling frequency in ms. Default: 2000
-  - \`--timeout <ms>\`: Max wait for answer in ms. Default: 0 (wait forever)
+Exits with code 1 on connection error.
 
 **\`cw answer "<answer>" --conversation-id <CONVERSATION_ID>\`**
 Answer a pending question. The conversation ID comes from the \`cw listen\` output.