p0-1: address review (drop unused thiserror dep, document kb-core reserve)

- Cargo.toml: remove `thiserror` from kb-config, kb-parse-types, kb-app
  (unused — none of those crates' src trees reference thiserror; CoreError
  in kb-core is the only consumer).
- kb-config keeps the `kb-core` dep with a one-line comment marking
  CoreError reserved for P1-* config-error wiring per the review thread.
- ids.rs: switch `validate_hex32` from a hand-rolled `matches!` byte range
  to `is_ascii_hexdigit()` so the hex check is the canonical idiom (and
  satisfies `clippy::manual_is_ascii_check` under `-D warnings`).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -283,7 +283,6 @@ dependencies = [
  "kb-core",
  "serde",
  "serde_json",
- "thiserror 2.0.18",
  "toml",
  "tracing",
  "tracing-appender",
@@ -311,7 +310,6 @@ dependencies = [
  "kb-core",
  "serde",
  "serde_json",
- "thiserror 2.0.18",
  "toml",
 ]
 
@@ -335,7 +333,6 @@ version = "0.1.0"
 dependencies = [
  "kb-core",
  "serde",
- "thiserror 2.0.18",
 ]
 
 [[package]]

crates/kb-app/Cargo.toml

@@ -11,7 +11,6 @@ description   = "Facade — orchestrates components for kb-cli/tui/desktop"
 kb-core              = { path = "../kb-core" }
 kb-config            = { path = "../kb-config" }
 anyhow               = { workspace = true }
-thiserror            = { workspace = true }
 serde                = { workspace = true }
 serde_json           = { workspace = true }
 tracing              = { workspace = true }

crates/kb-config/Cargo.toml

@@ -8,9 +8,9 @@ repository    = { workspace = true }
 description   = "Config schema + XDG path resolution"
 
 [dependencies]
+# kb-core::CoreError reserved for P1-* config errors
 kb-core      = { path = "../kb-core" }
 anyhow       = { workspace = true }
-thiserror    = { workspace = true }
 serde        = { workspace = true }
 serde_json   = { workspace = true }
 toml         = "0.8"

crates/kb-core/src/ids.rs

@@ -53,10 +53,7 @@ fn validate_hex32(s: &str) -> Result<(), CoreError> {
             s.len()
         )));
     }
-    if !s
-        .bytes()
-        .all(|b| matches!(b, b'0'..=b'9' | b'a'..=b'f' | b'A'..=b'F'))
-    {
+    if !s.bytes().all(|b| b.is_ascii_hexdigit()) {
         return Err(CoreError::InvalidId(format!(
             "non-hex character in {s:?}"
         )));

crates/kb-parse-types/Cargo.toml

@@ -10,4 +10,3 @@ description   = "Parser intermediate representations (no parser libs allowed)"
 [dependencies]
 kb-core    = { path = "../kb-core" }
 serde      = { workspace = true }
-thiserror  = { workspace = true }