fix(tui): add MCP list and remove slash entries

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53252.
Fixes PlatformNetwork/bounty-challenge#53255.

Summary

• register first-class /mcp-list with /mcp-ls alias for configured server discovery
• register first-class /mcp-remove with /mcp-rm alias next to the existing MCP slash commands
• route the new slash entries to the MCP manager, matching existing /mcp, /mcp-tools, and /mcp-auth behavior
• add registry and executor coverage for the new entries

Validation

• rustfmt +stable --check src/cortex-tui/src/commands/executor/dispatch.rs src/cortex-tui/src/commands/executor/tests.rs src/cortex-tui/src/commands/registry/builtin.rs src/cortex-tui/src/commands/registry/mod.rs
• cargo +stable test --package cortex-tui commands::registry::tests::test_specific_commands_registered
• cargo +stable test --package cortex-tui test_mcp_subcommands
• git diff --check

[greptile-apps]

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