refactor(workspace): P5 break hub cycles (tools↔engine, engine↔commands)

[yaohaowei0914]

Summary

Breaks the three narrow import cycles that would block extracting tools, engine, and commands into separate crates in Phase 6. Everything still compiles as one crate today; Phase 5 is purely a preparatory refactor.

Closes #74. Part of #68.

What changed

5a — Move Agent tool into engine/

• crates/claude-code-rs/src/tools/agent/ → crates/claude-code-rs/src/engine/agent/ (5 files)
• Tool registry updated to construct crate::engine::agent::AgentTool
• Agent tool is semantically an engine sub-process launcher; this is its natural home

5b — Extract HookRunner trait into cc-types

• New crates/cc-types/src/hooks.rs defines HookRunner trait + plain data types (HookEventConfig, HookEntry, HookOutput, PermissionOverride, PreToolHookResult, PostToolHookResult) + NoopHookRunner
• tools/hooks/mod.rs re-exports types and adds concrete ShellHookRunner
• QueryEngine, QueryEngineDeps, and ToolUseContext now hold Arc<dyn HookRunner> instead of importing tools::hooks directly
• Agent code fires SubagentStart/Stop hooks via ctx.hook_runner

5c — Invert engine → commands

• New crates/cc-types/src/commands.rs with CommandDispatcher trait, ParsedCommand type, NoopCommandDispatcher
• commands/mod.rs adds concrete DefaultCommandDispatcher
• process_user_input now takes &dyn CommandDispatcher; engine threads the dispatcher through

External call sites (main.rs, teams/runner.rs, web/handlers.rs) install the shell/default impls; tests use the no-op defaults.

Verification

Cycle-check greps (all return zero):

• grep -rn "use crate::engine" crates/claude-code-rs/src/tools/
• grep -rn "use crate::tools::hooks" crates/claude-code-rs/src/engine/
• grep -rn "use crate::commands" crates/claude-code-rs/src/engine/

Build & test:

• cargo build --release -p claude-code-rs — clean (two pre-existing warnings on unused session_id fields in web/handlers.rs)
• Targeted: engine::agent 59/59, engine::input_processing 6/6, tools::hooks 20/20, test_submit_local_command pass
• Full suite: 1226/1234; the 8 failures are pre-existing env-dependent tests (teams mailbox paths, daemon log path, Windows %TEMP%-tied tests, plugin paths, model_add pricing) — none touch hooks/agent/commands/input_processing

Test plan

• CI cargo build green
• CI cargo test green (or at least no new failures vs. rust-lite baseline)
• No new warnings
• Reviewer spot-check: Arc<dyn HookRunner> / Arc<dyn CommandDispatcher> wiring at external call sites is correct