fix(agentic): reject tool-invocation syntax in Write content generation

[bobleer]

Summary

• Add write_content_sanitizer to detect and strip DSML / nested tool-call artifacts from Write file-body generation.
• Retry streamed Write content generation once when the model returns tool-invocation syntax instead of raw file content; fail with guidance after the retry budget is exhausted.
• Validate and sanitize inline Write tool content before persisting to disk.

Context

Models sometimes emit DSML blocks (<｜｜DSML｜｜tool_calls>, <invoke>, etc.) or nested tool-call XML inside Write content streams. This PR prevents that syntax from being written to disk and gives the agent a clear retry/failure path.

Test plan

• cargo test -p bitfun-core write_content_sanitizer
• cargo test -p bitfun-core sanitization_strips_dsml
• cargo check -p bitfun-core