feat(plugins): add pre_approval_request / post_approval_response hooks

[teknium1]

Summary

Plugins can now observe dangerous-command approval events in real time, on both the CLI-interactive path and the async gateway path. This is the missing hook surface external tools (macOS allow/deny notifiers, Slack alerts, audit logs) need to build approval UX without forking Hermes or running a parallel gateway adapter.

Context: a community request — someone wants to ship a macOS menu-bar app that pops an allow/deny notification whenever Hermes needs approval. Today the only programmatic way is to register a full gateway platform adapter or hook into the TUI JSON-RPC bridge. This PR gives them a one-line plugin registration.

Changes

• hermes_cli/plugins.py: add pre_approval_request + post_approval_response to VALID_HOOKS
• tools/approval.py: fire both hooks from check_all_command_guards — CLI surface (around prompt_dangerous_approval) and gateway surface (around notify_cb + blocking event.wait loop). Single-chokepoint design — check_all_command_guards is the only real user-facing approval path on current main
• website/docs/user-guide/features/hooks.md: document both hooks with macOS-notification example
• tests/tools/test_approval_plugin_hooks.py: 5 tests (CLI once, CLI deny, plugin-crash resilience, gateway approve, gateway timeout)

Design

• Observer-only. Return values ignored — plugins cannot veto or pre-answer. Use pre_tool_call to block a tool before it reaches approval.
• Crash-safe. A crashing plugin cannot break the approval flow — invoke_hook already swallows per-callback errors, and the local wrapper adds a second layer that logs and swallows dispatch-layer errors too. Verified by test_plugin_hook_crash_does_not_break_approval.
• Lazy-imported. Approval module is imported very early, long before plugins are discovered — the helper lazy-imports hermes_cli.plugins.invoke_hook and no-ops if the plugin system isn't available.
• Both surfaces fire. surface="cli" for interactive CLI/TUI/ACP prompts, surface="gateway" for Telegram/Discord/Slack/Matrix/WhatsApp/BlueBubbles/etc.
• Timeout reported explicitly. post_approval_response gets choice="timeout" when the gateway prompt expires without a user response, distinct from "deny".

Validation

tests/tools/test_approval_plugin_hooks.py  5 passed
tests/tools/test_approval.py               full existing suite
tests/gateway/test_approve_deny_commands.py full existing suite
tests/hermes_cli/test_plugins.py           full existing suite
tests/hermes_cli/test_hooks_cli.py         full existing suite
──────────────────────────────────────────────────────
total                                      224 passed

Example usage (documented in hooks.md)

import subprocess

def notify_approval(command, description, session_key, **kwargs):
    subprocess.Popen([
        "osascript", "-e",
        f'display notification "{description}: {command[:80]}" with title "Hermes needs approval"',
    ])

def register(ctx):
    ctx.register_hook("pre_approval_request", notify_approval)