refactor: replace subprocess calls with library functions

Converts CLI subprocess calls to direct library function calls for
better integration and performance. Updates mdbook preprocessor to
use library functions instead of spawning external processes.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: nikomatsakis

Cargo.lock

@@ -5,9 +5,11 @@ edition = "2021"
 
 [dependencies]
 anyhow = "1.0.94"
+chrono = "0.4"
 clap = { version = "4.5.23", features = ["derive"] }
 mdbook = "0.4.52"
 regex = "1.11.1"
 rust-project-goals = { version = "0.1.0", path = "../rust-project-goals" }
+rust-project-goals-cli = { version = "0.1.0", path = "../rust-project-goals-cli" }
 semver = "1.0.23"
 serde_json = "1.0.133"