docs: note manual browser connection setup

[honor2030]

Summary

• add a short README note that browser-harness needs a connected browser before commands run
• point manual users to the two safer connection paths already covered in install.md
• avoid the unsafe bare --remote-debugging-port=9222 default-profile snippet

Test Plan

• git diff --check origin/main...HEAD
• uv run python -m compileall -q src tests

Closes #107

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Summary by cubic

Clarifies manual first-run setup in README so browser-harness connects to a browser before any commands run. Points to the two safe connection options from install.md (normal profile via chrome://inspect/#remote-debugging, or isolated profile with --remote-debugging-port=9222 --user-data-dir=<path> and BU_CDP_URL) and avoids the unsafe bare default-profile flag.

^{Written for commit 5203dba. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}