Add hook script tests

[roeimichael]

Problem

The hook scripts in hooks/ contain meaningful logic — build_envelope constructs event payloads with token estimation, format_statusline builds the statusline string, estimate_tokens performs chars-to-tokens conversion — but none of this is tested. The hooks are also excluded from pyright type checking.

What to do

Create tests/test_hooks.py that imports and tests the pure functions:

1. build_envelope — verify truncation, token estimation, payload structure
2. format_statusline — verify output format with various input combinations
3. estimate_tokens — verify calculation matches chars / chars_per_token_estimate
4. get_session_alert — test with mocked HTTP responses (collector up vs down)

Approach

These functions use only stdlib + httpx, so they can be imported and tested directly. The main() functions can be tested with stdin mocking.

Files to look at

• hooks/post_tool_use.py — build_envelope, estimate_tokens
• hooks/statusline.py — format_statusline
• hooks/_hook_config.py — config loading