chore: project maintenance, gitignore updates, and doc cleanup

[loki-hermes-agent]

Summary

Performs routine project maintenance to improve hygiene and reduce noise, without touching core functionality.

Changes:

• .gitignore: Added .pytest_cache/ and *.log to prevent untracked file noise.
• pyproject.toml: Removed aiohttp>=3.9 from root dev dependencies. The root package uses httpx, and aiohttp is already correctly managed as a runtime dependency in webhook-receiver/pyproject.toml.
• Documentation: Renamed docs/hermes-agent-design-final.md to docs/index.md to follow standard documentation conventions for a Table of Contents. Updated the reference in docs/hermes-implementation-plan.md.
• GitHub Config: Tracked .github/copilot-instructions.md.

Note: .github/workflows/opencode.yml was intentionally left untracked as the CI integration is incomplete and lacks the required repository secret.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Chore / Maintenance

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation