feat(mcp): make Labby-owned apps opt-in

[jmagar]

Summary

• make every Labby-owned MCP App UI opt-in by default, including the MCP Apps manager UI
• keep the root mcp_app control tool available text-only so admins can restore individual app surfaces
• automatically pass through allowed upstream MCP App tools/callbacks, _meta.ui, and native ui:// resources while ordinary raw upstream tools remain hidden in synthetic Code Mode
• preserve the same behavior for cached OAuth subject-scoped catalogs and protected-route/resource exposure rules
• update operator docs, OAuth docs, observability docs, config examples, and MCP source-of-truth instructions

Review fixes already applied

• fixed the mirrored CodeModeAppState default so runtime state agrees with persisted config
• removed the manager UI's previous always-on bypass
• fixed tests that implicitly depended on the old enabled-by-default policy
• kept handlers_tools and peer_contract descriptor construction in lockstep
• fixed Clippy's derivable-default finding for McpAppsConfig

Validation

• cargo test -p labby-runtime
• cargo test -p labby-gateway mcp_app_
• cargo test -p labby mcp_app_
• cargo test -p labby --features proxy-testkit list_tools_passes_through_subject_scoped_oauth_mcp_apps_in_code_mode
• targeted root/protected/resource catalog regressions for upstream MCP Apps
• just docs-check
• just check
• cargo clippy -p labby-runtime --all-features --all-targets -- -D warnings
• cargo clippy -p labby-gateway --all-features --all-targets -- -D warnings
• cargo clippy -p labby --all-features --all-targets --locked -- -D warnings
• changed-crate rustdoc doctests with RUSTDOCFLAGS=-D warnings
• git diff --check

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