chore(release): prepare ksadk 0.6.9 public candidate

[AgentArcLab]

Summary

Prepare the reviewed KsADK 0.6.9 public candidate from the internal clean export.

This candidate:

• keeps docs-site/ as the public GitHub Pages source of truth;
• removes stale public docs/ snapshots, internal tests/scripts, and legacy zread docs deploy remnants from the public tree;
• updates README/docs/release gates for trusted GitHub workflow publishing;
• bundles @kingsoftcloud/ksadk-web@0.2.18 through the registry path during preflight.

Verification

Run in .worktrees/public-main:

• make public-preflight passed
  • version gate passed: ksadk 0.6.9 > PyPI 0.6.8, agentengine-sdk-python 0.6.9 > PyPI 0.6.4
  • public source audit: 912 checked, 0 violations
  • synced @kingsoftcloud/ksadk-web@0.2.18 from npm registry
  • public tests: 15 passed
  • Fumadocs static build: 181 pages generated
  • wheel/sdist build passed
  • twine check dist/* passed
  • wheel audit: 487 checked, 0 violations
  • sdist audit: 545 checked, 0 violations
• KSADK_APPROVED_SOURCE_COMMIT=1f3c0d844dc4df2630c8e1d410bcd9f7c9b595cc make public-publish-gate passed

Release Boundary

Do not publish from local machines. npm, PyPI, and GitHub Pages must be published by trusted GitHub workflows only.