feat(secret-tools): add agent-facing keyring secret tools

[GregKinne]

Summary

• Adds a builtin secret_tools plugin with set_secret, get_secret, delete_secret, and list_secrets.
• Backs all values with code_puppy.secret_store, so persistence uses the OS secret manager or hardened fallback.
• Injects agent instructions to prefer secret tools over plaintext files, logs, memory notes, or docs.
• Maintains a best-effort secret-name index without exposing values in list output.

Tests

• ruff check code_puppy/plugins/secret_tools tests/plugins/test_secret_tools.py
• pytest tests/plugins/test_secret_tools.py tests/plugins/test_register_agent_tools_hook.py tests/test_tools_registration.py tests/test_tools_init_full_coverage.py