Enforce effective MCP object allowlists

[lucarlig]

Summary

• Reject tool calls absent from the selected backend effective tool allowlist before plugins or backend connection.
• Apply the same fail-closed check to prompts and resource URIs.
• Add allowed_resource_uris to config/schema and document the new routing behavior.
• Cover rejection-before-connection and old-config deserialization with regression tests.

Integration dependency

Draft until the ContextForge control-plane publisher populates allowed_resource_uris; it currently publishes resource display names only, so resource reads would otherwise fail closed.

Testing

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo nextest run --locked --workspace (211 passed)
• cargo deny check advisories licenses
• cargo build --locked --workspace
• cargo shear --check-test-targets --deny-warnings --locked
• mdbook build _context/wiki
• mdbook test _context/wiki
• git diff --check

[dawid-nowak]

You might as well close this one. It is done as part of #114

[lucarlig]

closed is part of #118