Add browser probe DOM assertions

[chubes4]

Summary

• Add repeatable assert= support to wordpress.browser-probe for selector state, count, text, attribute, console-error, and page-error assertions.
• Include assertion summaries/results in command JSON and files/browser/summary.json, with non-advisory failures failing after artifacts are written.
• Add assertion smoke coverage and refresh command metadata/docs.

Fixes #605.

Verification

• npm run build
• npm run command-registry-smoke
• npm run task-input-contract-smoke
• npm run browser-probe-artifact-smoke
• npm run browser-probe-assertions-smoke
• npm run agent-sandbox-code-smoke

Known blocker

• npm run check currently fails at recipe-interruption-artifacts-smoke with missing manifest entry files/runtime-evidence/run-attestation.json. This reproduces outside the browser-probe assertion path on current origin/main; targeted browser-probe checks pass.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (gpt-5.5)
• Used for: Drafted the browser-probe assertion implementation, smoke coverage, docs, and verification notes for Chris to review and test.