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.lock

@@ -492,6 +492,7 @@ dependencies = [
  "http-body-util",
  "hyper",
  "hyper-util",
+ "json-five",
  "json5",
  "libc",
  "log",
@@ -1531,6 +1532,16 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "json-five"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "865f2d01a4549c1fd8c60640c03ae5249eb374cd8cde8b905628d4b1af95c87c"
+dependencies = [
+ "serde",
+ "unicode-general-category",
+]
+
 [[package]]
 name = "json5"
 version = "1.3.1"
@@ -3384,6 +3395,12 @@ version = "2.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dbc4bc3a9f746d862c45cb89d705aa10f187bb96c76001afab07a0d35ce60142"
 
+[[package]]
+name = "unicode-general-category"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0b993bddc193ae5bd0d623b49ec06ac3e9312875fdae725a975c51db1cc1677f"
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.24"