docs: Consolidate steering files into AGENTS.md and update actions

[Unril]

Changes

Consolidate steering files into root AGENTS.md:

• Move content from .kiro/steering/{product,tech,structure}.md into a single AGENTS.md at the repo root
• Remove AGENTS.md from .gitignore

Update GitHub Actions:

• actions/dependency-review-action v4.9.0 → v5.0.0 (Node.js 24 runtime bump)
• github/codeql-action v4.35.3 → v4.35.4

Corrections during consolidation (verified against source):

• Fix glaze serialization note: kotlin-lsp requires null values, so skip_null_members is false (old docs said 'skip null optionals')
• Clarify Android variant selection: picks 'first non-test, typically debug' (matches pick_android_variant in gradle.hpp)
• Expand CLI flag documentation (--log-level, --version, --save-gradle-output, --jvm-target)
• Note that tests are auto-discovered via file(GLOB) in CMakeLists.txt
• Mention resolve-action-shas.sh's Python implementation file

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

OpenSSF Scorecard

Package	Version	Score	Details
actions/github/codeql-action/upload-sarif	68bde559dea0fdcac2102bfdf6230c5f70eb485e	Unknown	Unknown

Scanned Files

• .github/workflows/scorecard.yml

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.79%. Comparing base (6f3f962) to head (056e019).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #14   +/-   ##
=======================================
  Coverage   89.79%   89.79%           
=======================================
  Files          13       13           
  Lines         794      794           
  Branches      115      115           
=======================================
  Hits          713      713           
  Misses         52       52           
  Partials       29       29           

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