Allow pwd and date

[justinmclean]

What this changes

Both were refused in a live run: the agent asked for pwd and date before doing anything else, got capability_denied for each, and spent a round trip rediscovering that python3 -c "import os; print(os.getcwd())" and python3 -c "import datetime" are the permitted spellings. At local model rates that is over a minute of wall clock each, plus the denial and the workaround in every subsequent prompt for the rest of the loop.

Neither is a send vector, which is where the deny list draws its boundary, and both are strictly read-only. They belong with the other verification primitives.

Also drops the hardcoded "32-entry" from _check_config_source's docstring, which this change would have made wrong and which the next one would have made wrong again; the count was never the point of the example.

Checklist

• One work item only — nothing unrelated bundled in
• make check is green locally
• Tests cover the change, including refusal paths (the negative cases
  are the product)
• No test, acceptance criterion, or check was weakened to get green
• No changes under specs/ (spec changes go through an issue first)
• Docs updated where behaviour changed (README / USAGE.md)
• Commit subjects are imperative; Generated-by: trailer present if an
  agent wrote the change