Tool system: Align chat sin_edit semantics with MCP sin_edit

[Delqhi]

Problem

sin_edit in chat is a naive string replace. sin_edit in MCP is a surgical AST/anchor/string editor. Same name, different behavior — prompts break when switching between sin-code chat and sin-code serve.

What to do

1. Unify sin_edit implementation:
   • Use the same internal/edit package for both chat and MCP
   • Support: anchor mode, symbol mode, string mode
   • Syntax validation before write
2. Deprecate chat's naive string replace
3. Add sin_replace chat tool for backward compatibility (simple string replace)
4. Update all agent profiles to prefer sin_edit with anchors
5. Tests ensure identical behavior across surfaces

Acceptance criteria

• Chat and MCP sin_edit use same engine
• sin_replace for backward compatibility
• Agent profiles updated
• Tests for both surfaces
• Migration note in CHANGELOG