docs: contribution guide + 5 good-first-issues

[sepehr-safari]

Summary

Expands CONTRIBUTING.md with:

• Architecture Overview — core data flow diagram, key modules table, browser-safe vs Node-only
• Testing Guide — how to run tests, writing tests, test patterns
• Contributing Scenarios — step-by-step guide for adding a new scenario
• Contributing Detection Rules — step-by-step guide for adding a new failure rule
• Good-First-Issue Workflow — guidance for new contributors

Also creates 5 good-first-issues on GitHub (#104–#108):

• Add a new scenario: firmware-update-success #104: Add a new scenario: firmware-update-success
• Add a new detection rule: REPEATED_BOOT_NOTIFICATION #105: Add a new detection rule: REPEATED_BOOT_NOTIFICATION
• Improve test coverage for the summarizer module #106: Improve test coverage for the summarizer module
• Add --verbose flag to inspect command #107: Add --verbose flag to inspect command
• Add a new scenario: meter-value-zero #108: Add a new scenario: meter-value-zero

Closes #90

Checklist

• CONTRIBUTING.md expanded with architecture overview, setup guide, code org guide, testing guide, scenario/detection contribution guides
• 5 good-first-issues created with good-first-issue label
• Good-first-issues have clear instructions, acceptance criteria, and code pointers
• No maintainer email in any committed file
• No changeset needed (docs-only)