docs(secops): add Agentic Detection Engineering guide and tool documentation

[dandye]

Summary

Fast follow to #305: adds dedicated documentation, user guide, and Sphinx documentation updates for Google SecOps Agentic Detection Engineering (ADE).

This includes:

• A comprehensive end-to-end guide on Agentic Detection Engineering in Google SecOps.
• Visual instructions with screenshot for enabling Synthetic Data Visibility in the SecOps Web UI.
• Detailed reference and walkthrough examples for the 5 newly added ADE FastMCP tools.
• Skill registration and table-of-contents / navigation links.

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Changes

1. Dedicated ADE Guide (docs/agentic_detection_engineering.md)

• Explains the end-to-end detection lifecycle: Threat Intel Ingestion $\rightarrow$ TDO Generation $\rightarrow$ Synthetic Simulation $\rightarrow$ Sandboxed Rule Coverage Evaluation (LRO) $\rightarrow$ Gap Analysis $\rightarrow$ Candidate YARA-L 2.0 Rule Generation $\rightarrow$ Verification & Deployment.
• Includes step-by-step instructions on enabling Synthetic Data Visibility in the Google SecOps UI (Settings > User Preferences > Synthetic Data Visibility > Show synthetic test data).
• Embeds the UI screenshot (docs/img/synthetic_data_visibility.png).
• Documents the /security:detect skill (detection-engineering-coverage-evaluation).
• Outlines best practices, human-in-the-loop authorization gates, and prompt-injection defenses when consuming external threat reports.

2. SecOps MCP Server Documentation (docs/servers/secops_mcp.md)

• Added Detection Engineering Agent Tools section detailing:
  • generate_threat_detection_opportunity
  • generate_synthetic_events
  • evaluate_rule_coverage_long_running
  • get_operation
  • generate_rules
• Added Example 9: Agentic Detection Engineering Workflow demonstrating an end-to-end conversation from threat description to synthetic events, LRO evaluation, and candidate rule generation.

3. Extension & Navigation Updates

• docs/google_secops_extension.md: Registered Skill 6 (detection-engineering-coverage-evaluation).
• docs/toc.md: Added [Agentic Detection Engineering](agentic_detection_engineering.md) to the Table of Contents.
• docs/index.md: Added quick link and description for Agentic Detection Engineering.

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Verification

• Built Sphinx HTML documentation locally (make -C docs html):

  make -C docs html
  # build succeeded, images copied: img/synthetic_data_visibility.png
  # HTML pages generated in docs/_build/html

• Verified image rendering and cross-references.
• Zero secrets committed (isolated .envrc and virtualenv ignored).

TAG=agy
CONV=b434fecb-6fd3-4841-b909-49d23ef70fa3