Add WebhookProcessor, public API exports, and handler tests

[Keramikus-97]

Summary

The repo had all the building blocks (comment parsing, config, GitHub client, webhook handler) but no orchestration layer tying them together, and __init__.py didn't export any public symbols.

New handler.py — WebhookProcessor: End-to-end webhook processing that chains signature verification → parse_raw → extract_commands → acknowledgement reaction:

async with WebhookProcessor(config, webhook_secret="optional") as proc:
    result = await proc.process(event_header, body, signature)
    # result.commands  — list[ParsedCommand]
    # result.acknowledged — bool (eyes reaction sent)
    # result.skipped_reason — "" | "invalid_signature" | "unsupported_event" | "no_commands"

Acknowledgement failures (e.g. API 500) are caught and logged so they never bubble up.

__init__.py: Exports all public symbols (Config, GitHubClient, WebhookProcessor, ParsedCommand, etc.) via __all__.

Tests: 9 new tests covering command extraction, signature pass/fail, unsupported events, acknowledge failures, and multi-command payloads.

Note: A CI workflow (.github/workflows/ci.yml) was also prepared locally but could not be pushed due to missing workflow scope on the OAuth token. It runs ruff check/format + pytest --cov across Python 3.10–3.12. You can add it manually or grant the workflow scope.

Link to Devin session: https://app.devin.ai/sessions/306b14b0580b4a1d830a4afc1c2dfd86
Requested by: @Keramikus-97