Rewrite README.md with comprehensive documentation

[devin-ai-integration]

Summary

Complete rewrite of README.md from a ~264-line demo-oriented quickstart into a ~550-line comprehensive reference document. No code changes — docs only.

Key additions over the old README:

• Table of contents with anchor links
• CI status badge
• Architecture diagram (ASCII art)
• Detailed "Getting Started" with prerequisites, two install options, and server verification
• Full API endpoint reference (health, employees, reports, audit) with request/response field tables
• Severity classification table showing GREEN/YELLOW/RED triggers, exit codes, pre-commit behavior, and CI behavior
• Exit codes 10 (GREEN) and 11 (YELLOW) — previously undocumented
• Slack notification message structure breakdown
• CI/CD pipeline phases table and "how to fix a failing check" instructions
• Contract test class inventory (11 classes)
• Wiki section linking to all 14 docs/wiki/ pages from PR Add comprehensive documentation wiki for Docs Drift Guard #10
• Tech stack table with versions

Corrections to old README:

• Pre-commit hook now correctly described as "imports the FastAPI app directly" (no temp server) — matches current scripts/pre-commit-drift-check.sh
• .pre-commit-config.yaml correctly described as Ruff-only (old README implied it ran drift checks)
• Clone URL updated from <repo-url> placeholder to actual GitHub URL
• Project root folder name updated from docs-drift-guard-demo/ to DevinTestRepo/

Updates since initial revision (review feedback):

• Fixed test class count from 12 → 11 (in prose text at line 225 and project structure comment at line 493) — the table correctly listed only 11 classes
• Corrected severity classification for enums: "enum values added (to existing enum)" moved from YELLOW to GREEN, matching drift_detect.py:383-396; YELLOW now reads "enum values removed, new enum constraint added"

Review & Testing Checklist for Human

• Verify exit codes 10/11 match code: The README documents exit codes 10 (GREEN) and 11 (YELLOW) based on drift_guard/guard.py lines 142-147. Confirm these are correct and intentionally undocumented before.
• Verify severity classification against drift_detect.py: The GREEN/YELLOW/RED trigger descriptions were corrected for enum handling. Cross-check the full severity table against drift_guard/drift_detect.py (especially lines 369-426) to confirm all triggers are accurately categorized.
• Spot-check API endpoint fields: The Employee request/response field tables were derived from app/main.py models. Verify bio, dob, language are actually returned in responses and that the required/optional flags are accurate.
• Verify wiki links resolve: The Wiki section links to 14 pages in docs/wiki/. Click a few to confirm they render correctly on GitHub after merge.

Suggested test plan: Render the README on a GitHub branch preview and click through anchor links and wiki links. Optionally, start the server locally and compare the documented endpoints/fields against actual responses.

Notes

• Link to Devin run: https://app.devin.ai/sessions/521b2ec6fa2e4c319074825d96c1704b
• Requested by: @justinemach

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