refactor(agent): replace XML api_call with native function calling

Migrate the Telegram bot agent from an XML tag hack (<api_call>) to
OpenAI-native function calling via CallWithRequestFull.

Key changes:
- mcp/interface.go: add parseMCPResponseFull to clientHooks interface
- mcp/client.go: route callWithRequestFull through hooks for overridability
- mcp/claude_client.go: override parseMCPResponseFull for Claude response
  format (tool_use blocks instead of choices[].message.tool_calls)
- telegram/agent/agent.go: rewrite Run() to use CallWithRequestFull;
  define api_request tool with JSON Schema; implement tool-call loop
  with role="tool" result messages; remove XML parsing entirely
- telegram/agent/apicall.go: remove parseAPICall (dead code)
- telegram/agent/prompt.go: simplify — remove XML format instructions,
  replace with concise api_request tool usage instructions
- telegram/agent/agent_test.go: rebuild all tests using LLMResponse
  objects; add TestNarrationStructurallyImpossible, TestOnChunkCalledWithFinalReply,
  TestToolCallIDPropagated; remove XML-specific tests

Architecture advantage: with native function calling, the LLM returns
EITHER ToolCalls OR Content — never both. Narration is now structurally
impossible at the protocol level, not just enforced by prompt rules.

All 11 agent tests pass. mcp package tests pass.

telegram/agent/prompt.go

@@ -13,35 +13,22 @@ func BuildAgentPrompt(apiDocs, userID string) string {
 - All API calls are made on behalf of this user
 - When asked "which user / username / email" — answer with this user ID directly, no API call needed
 
-## Tool: api_call
-
-When you need to call the API, your ENTIRE response must be ONLY the tag — nothing else:
-<api_call>{"method":"GET","path":"/api/xxx","body":{}}</api_call>
-
-When you have a final answer (no more API calls needed), reply with plain text — NO tag at all.
-
-ABSOLUTE RULES — violation = broken product:
-- 【ZERO NARRATION】Your response is EITHER the api_call tag alone OR a final text reply. NEVER both except api_call at the very end.
-- NEVER output ANY text before an api_call tag. No "好的", no "现在", no "我将", no "Let me", no "I will", no "正在", no "Creating...", no ellipsis, NOTHING.
-- NEVER more than one <api_call> tag per response
+## Tool: api_request
+Use the api_request tool to call the NOFX REST API:
 - method: "GET" | "POST" | "PUT" | "DELETE"
+- path: API path; query params go in the path: /api/positions?trader_id=xxx
 - body: JSON object (use {} for GET requests)
-- query parameters go in the path: /api/positions?trader_id=xxx
 
 ## NOFX API Documentation
 
 %s
 
 ## Behavior Rules
-1. 【SILENT ACTION】When you need to call an API: output ONLY the <api_call> tag. Zero words before it.
-2. Only ONE <api_call> tag per response, always alone with nothing else
-3. After getting an API result, decide: call another API (output tag only) or give final reply (text only)
-4. If the API returns success (2xx), the operation succeeded — do not retry
-5. Reply in the same language the user used (中文→中文, English→English)
-6. Keep final replies concise — show results, not process
-7. Ask for ALL missing required info in ONE message — never ask one field at a time
-8. When user provides enough info, act immediately — no confirmation needed
-9. Be decisive — infer intent from context, use schema to fill in smart defaults
+1. Reply in the same language the user used (中文→中文, English→English)
+2. Keep final replies concise — show results, not process
+3. Ask for ALL missing required info in ONE message — never ask one field at a time
+4. When user provides enough info, act immediately — no confirmation needed
+5. Be decisive — infer intent from context, use schema to fill in smart defaults
 
 ## Verification Rule (CRITICAL)
 After ANY PUT or POST that creates or modifies a resource:
@@ -56,7 +43,6 @@ After ANY PUT or POST that creates or modifies a resource:
 - "AI model not enabled": tell user to enable the model first via PUT /api/models
 - "Exchange not enabled": tell user to enable the exchange first
 - 5xx: server error, ask user to try again
-- stream interrupted / unavailable: apologize briefly and ask user to retry
 
 ## Account State (injected at conversation start)
 At the start of each new conversation, a [Current Account State] block is provided with: