perf/fix: prompt_pipeline lru_cache leak + clear_cache stale; prompt Any hint

prompt_pipeline._build_cached had @lru_cache(maxsize=1) on an instance method
that is called once per fresh PromptPipeline: the cache never helped (new
instance each turn → miss) and (a) leaked the instance via the bound-method
cache and (b) made clear_cache() ineffective (lru_cache returned a stale
string after sections were cleared). Removed the decorator so each build()
rebuilds and clear_cache actually invalidates.

prompt.py: drop a stray `: Any` annotation that referenced an unimported name.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: QUSETIONS

minicode/prompt.py

@@ -200,7 +200,7 @@ def _build_skills():
     # MCP servers section
     mcp_servers = extras.get("mcpServers", [])
     if mcp_servers:
-        def _server_line(server: Any) -> str:
+        def _server_line(server) -> str:
             # Guard each entry so one malformed server (missing keys / non-dict)
             # can't crash the whole system-prompt build.
             if not isinstance(server, dict):

minicode/prompt_pipeline.py

@@ -13,7 +13,6 @@
 
 from __future__ import annotations
 
-import functools
 import hashlib
 import time
 from dataclasses import dataclass, field
@@ -102,7 +101,6 @@ def build(self) -> str:
         """Assemble the full system prompt with cache boundary marker."""
         return self._build_cached()
 
-    @functools.lru_cache(maxsize=1)
     def _build_cached(self) -> str:
         """Assemble the full system prompt with cache boundary marker."""
         parts: list[str] = []