Add runtime tool policy opt-in

[chubes4]

Summary

• Adds a generic policy/filter primitive that excludes caller-provided runtime tools by default unless explicitly opted in by name/category, allow-mode policy, allow_only, or mandatory policy.
• Documents the runtime-tool visibility boundary separately from parameter sourcing so sensitive required parameters remain auditable through explicit client_context_bindings.
• Expands smoke coverage for allow mode, deny mode, allow_only, explicit deny composition, non-runtime tools, and sensitive ambient context handling.

Verification

• php tests/tool-policy-contracts-smoke.php
• php tests/tool-runtime-smoke.php
• composer test

Closes #254.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (gpt-5.5)
• Used for: implementation draft, tests, and verification