fix: add mcp-list slash command

[SlaVKsVolks]

Summary

• adds /mcp-list as a dedicated MCP slash command
• routes it to the existing MCP manager, matching the other MCP slash-command entry points
• covers registry discovery and command execution with focused tests

Fixes PlatformNetwork/bounty-challenge#53252.

Verification

• rustfmt --edition 2024 --check src/cortex-tui/src/commands/registry/builtin.rs src/cortex-tui/src/commands/registry/mod.rs src/cortex-tui/src/commands/executor/dispatch.rs src/cortex-tui/src/commands/executor/tests.rs
• cargo test -p cortex-tui commands::registry::tests::test_specific_commands_registered
• cargo test -p cortex-tui commands::executor::tests::test_mcp_list_opens_mcp_manager
• cargo check -p cortex-tui
• git diff --check

Note: full cargo fmt --check --package cortex-tui is currently blocked by a pre-existing formatting diff in src/cortex-tui/src/question.rs, outside this PR.

[greptile-apps]

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