fix: clarify MCP slash command labels

[oz-for-oss]

Closes #9880

Summary

• Clarified /add-mcp so the slash-command menu says it adds a server via the MCP settings page.
• Renamed the bundled file-config MCP skill display name to agent-add-mcp while keeping its existing description.
• Added regression tests for both updated labels.

Validation

• cargo fmt
• git --no-pager diff --check
• cargo test -p ai test_parse_with_front_matter
• Lightweight direct metadata check for /add-mcp and bundled agent-add-mcp strings

Validation limitations:

• cargo test -p warp add_mcp --lib was attempted twice, including with CARGO_BUILD_JOBS=1, but the sandbox killed the app crate compile with SIGKILL.
• CARGO_BUILD_JOBS=1 cargo clippy --workspace --all-targets --all-features --tests -- -D warnings was attempted. After installing missing libclang, it remained blocked because the sandbox cannot access the private warp-channel-config repository needed to generate release-bundle channel config files for all-features clippy.

Notes

• The implementation is intentionally scoped to the requested minimal UI/text disambiguation.