fix: add mcp remove slash commands

[SlaVKsVolks]

Summary

• adds /mcp-remove and /mcp-rm as dedicated MCP removal slash entries
• routes both commands to the existing MCP manager so removal stays centralized
• adds focused registry and executor coverage

Fixes PlatformNetwork/bounty-challenge#53255.

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_remove_commands_open_mcp_manager
• cargo check -p cortex-tui
• git diff --check

[greptile-apps]

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