[workspace-split] Phase 8: Thin root bin crate + final measurements

[yaohaowei0914]

Part of #68. Target branch: V1. Depends on #76.

Scope

Reduce crates/claude-code-rs/ to a thin bin crate containing only the entry point.

Expected final contents:

• src/main.rs
• src/cli.rs
• src/shutdown.rs
• src/startup/ (or extract to cc-startup if big enough)
• Thin glue pulling together all cc-* crates via feature flags

Tasks

• Audit src/ for any remaining non-entry-point code; file dangling-code follow-up issues
• Verify root Cargo.toml uses [workspace.dependencies] for all shared deps
• Add a top-level CONTRIBUTING.md section explaining the crate layout
• Update CLAUDE.md to document the workspace structure
• Close [workspace-split] Epic: monolithic crate → Cargo workspace #68 with the final build-time measurements table

Acceptance

• claude-code-rs crate <1,500 LOC
• Full phase-by-phase measurements table attached to epic
• Target met: incremental build after editing any tool <3s