docs: correct install and authority command guidance

[HarperZ9]

Summary

• replace false PyPI install guidance with the GitHub v0.1.0 wheel path plus SHA256 verification
• document the current source-checkout path for post-v0.1.0 durable-authority features
• add the safe accountable-surface-authority --help surface and keep authority actions out of examples
• update the browser optional-extra install to use the source checkout

Verification

• git diff --check
• documented source install: python -m pip install -e ".[test]" built and installed accountable-surface-0.1.0 in a disposable venv
• accountable-surface-authority --help with the documented sibling PYTHONPATH -> exit 0; subcommands: revoke, recover-precommit, mark-ambiguous, doctor
• python -m accountable_surface.authority_cli --help from source -> exit 0; no authority grant/revoke/recovery action was run
• python -m pytest -> 457 passed in 13.67s; Windows emitted a pytest temp cleanup warning after success
• node --test web/*.test.mjs -> 5 passed
• checked PyPI: accountable-surface returned 404
• checked GitHub release v0.1.0 assets and SHA256SUMS.txt includes accountable_surface-0.1.0-py3-none-any.whl