feat(M6): implement hooks system with PreToolUse, PostToolUse, and SessionStart events

Milestone M6 implementation adds a comprehensive hook system that allows
users to run custom scripts at various lifecycle events.

New crate: crates/platform/hooks
- HookEvent enum with multiple event types:
  * PreToolUse: fires before tool execution, can deny operations (exit code 2)
  * PostToolUse: fires after tool execution
  * SessionStart: fires at session start, can persist env vars
  * SessionEnd, UserPromptSubmit, PreCompact (defined for future use)
- HookManager for executing hooks with timeout support
- JSON I/O: hooks receive event data via stdin, can output to stdout
- Hooks located in .owlen/hooks/{EventName}

CLI integration:
- All tool commands (Read, Write, Edit, Glob, Grep, Bash, SlashCommand)
  now fire PreToolUse hooks before execution
- Hooks can deny operations by exiting with code 2
- Hooks timeout after 5 seconds by default

Tests added:
- pretooluse_can_deny_call: verifies hooks can block tool execution
- posttooluse_runs_parallel: verifies PostToolUse hooks execute
- sessionstart_persists_env: verifies SessionStart can create env files
- hook_timeout_works: verifies timeout mechanism
- hook_not_found_is_ok: verifies missing hooks don't cause errors

All 63 tests passing.

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

Co-Authored-By: Claude <noreply@anthropic.com>

crates/app/cli/Cargo.toml

@@ -17,6 +17,7 @@ tools-bash = { path = "../../tools/bash" }
 tools-slash = { path = "../../tools/slash" }
 config-agent = { package = "config-agent", path = "../../platform/config" }
 permissions = { path = "../../platform/permissions" }
+hooks = { path = "../../platform/hooks" }
 futures-util = "0.3.31"
 
 [dev-dependencies]