refactor: extract tool call parsing into parsing.rs

Move parse_leaked_tool_calls, strip_leaked_artifacts, and their
helpers (normalize_xml_tags, parse_qwen_tag, parse_xml_tool_call,
parse_json_tool_call) from agent.rs into their own module.

These functions have zero dependency on Agent or ContextState —
they're pure text parsing. All 4 existing tests move with them.

Reduces agent.rs by ~200 lines.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-Authored-By: Qwen 3.5 27B <noreply@qwen.ai>

poc-agent/src/main.rs

@@ -41,6 +41,7 @@ mod dmn;
 mod journal;
 mod log;
 mod observe;
+mod parsing;
 mod tools;
 mod tui;
 mod types;