config_writer: json5 round-trip editing via json-five

Surgical edits to ~/.consciousness/config.json5 that preserve comments,
whitespace, trailing commas, and unquoted identifier keys on round-trip.

Uses json-five's rt::parser module — a real JSON5 parser with AST
mutation + faithful serialization back. set_scalar(section, key, literal)
locates or creates the target, replaces the value; set_learn_threshold
is a convenience for the common F-screen use case.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/lib.rs

@@ -42,6 +42,7 @@ pub mod subconscious;
 
 // Unified configuration
 pub mod config;
+pub mod config_writer;
 
 // Session state
 pub mod session;