PII redaction: build_session_context_prompt() now checks the plugin
registry's pii_safe flag in addition to the hardcoded _PII_SAFE_PLATFORMS
frozenset. Plugin platforms that set pii_safe=True (e.g. phone-based
messaging bridges) get their user IDs redacted before LLM context.
Token empty warnings: the empty-token diagnostic at config load now
checks the plugin registry's required_env when a platform isn't in the
hardcoded _token_env_names dict. Catches 'enabled but empty' for
plugin platforms too.