chore: bump default worai to 6.20.10

[ziodave]

Summary

• bump the default worai_version to 6.20.10
• keep README, usage docs, and the action behavior spec aligned
• assert the public action default in the installer test

Validation

• ./tests/run-worai.sh, 18 passed
• ./tests/install-worai.sh, 7 passed
• ./tests/install-playwright.sh, 8 passed
• ./tests/resolve-cache-key.sh, 10 passed
• ./tests/run-graph-kpis.sh, 23 passed
• actionlint
• git diff --check

Release note

this PR does not create a release or move the v6 alias. the new default becomes live only after a subsequent patch release.

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