Improve agent Read/Edit reliability with clearer guidance and larger read budget

[bobleer]

Summary

• Raise default Read output cap from 16k to 64k characters and align READ_MAX_TOOL_RESULT_CHARS storage limit so agents receive enough file context before Edit.
• Tighten agent prompts and Edit/Read tool descriptions to require verbatim copy from Read results (preserve indentation, avoid reformatting HTML/CSS/JS).
• Improve Edit not-found diagnostics with actionable re-read guidance and a single closest snippet instead of multiple noisy matches.

Test plan

• cargo test -p bitfun-core edit_tool
• cargo test -p tool-runtime
• cargo check -p bitfun-core -p tool-runtime