chore(deps): remove unused dependencies (cargo-machete cleanup) (#234)

Drop dependencies confirmed unused by cargo-machete in three crates:

- plumb-core: thiserror, miette, ahash
- plumb-mcp: tracing
- plumb-codegen: serde_json, tracing

All six were verified absent from each crate's source tree. Workspace
build, full nextest suite, and just validate all pass.

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

Author: aram-devdocs

Cargo.lock

@@ -18,10 +18,8 @@ exclude = ["AGENTS.md", "CLAUDE.md"]
 plumb-core = { workspace = true }
 plumb-config = { workspace = true }
 indexmap = { workspace = true }
-serde_json = { workspace = true }
 toml = { workspace = true }
 thiserror = { workspace = true }
-tracing = { workspace = true }
 
 [dev-dependencies]
 insta = { workspace = true }

crates/plumb-codegen/Cargo.toml

@@ -25,10 +25,7 @@ test-fake = []
 serde = { workspace = true }
 serde_json = { workspace = true }
 schemars = { workspace = true }
-thiserror = { workspace = true }
-miette = { workspace = true }
 indexmap = { workspace = true }
-ahash = { workspace = true }
 palette = { workspace = true }
 rayon = { workspace = true }
 

crates/plumb-core/Cargo.toml

@@ -27,7 +27,6 @@ axum = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 thiserror = { workspace = true }
-tracing = { workspace = true }
 subtle = { workspace = true }
 
 [dev-dependencies]