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>

Cargo.toml

@@ -30,6 +30,7 @@ log = "0.4"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 json5 = "1.3"
+json-five = "0.3"
 
 ratatui = { version = "0.30", features = ["unstable-rendered-line-info"] }
 tui-markdown = { git = "https://github.com/koverstreet/tui-markdown", subdirectory = "tui-markdown" }