ci: enhance GitHub Actions workflow

[krisoye]

Summary

Enhances the existing CI/CD workflow with improved test coverage tracking, non-blocking lint checks, and better build reliability.

Changes

• Add workflow_dispatch trigger for manual workflow runs
• Make lint job non-blocking with continue-on-error: true (lint failures are reported but don't fail the build)
• Add fail-fast: false to matrix strategy so all Python versions are tested even if one fails
• Add pip caching for faster CI builds
• Add 80% coverage threshold to fail builds if coverage drops below acceptable level

Test Plan

• All 266 tests pass locally
• Coverage is 84% (above 80% threshold)
• Lint checks pass with ruff
• GitHub Actions workflow runs successfully after merge

Notes

The existing test suite already has comprehensive mocking for external dependencies:

• Anthropic API calls are mocked in LLM processor tests
• Tesseract OCR is mocked in OCR tool tests
• Settings and configuration are mocked throughout

This means CI can run without external API keys or system dependencies.

---

Generated with Claude Code