Add zarvis harness hooks

[edwin-zvs]

Summary

• add explicit opt-in zarvis hook config via AGENTD_ZARVIS_HOOKS_CONFIG or AGENTD_ZARVIS_HOOKS_JSON
• emit notification hooks for session_start, user_prompt_submit, pre_tool_use, post_tool_use, tool_approval_request, and session_stop from headless and interactive zarvis paths
• add separate mutating hooks: user_prompt_mutate can rewrite the prompt and pre_tool_use_mutate can rewrite tool args before display/approval/execution
• document the hook config and add focused tests for config loading, stdin JSON delivery, and mutating stdout

Refs #52

Tests

• cargo test -p agentd-adapter-zarvis hooks -- --nocapture
• cargo check -p agentd-adapter-zarvis
• cargo build