Align MCP existing server guidance

[GreyforgeLabs]

Summary

• Route MCP "already exists" messages through a shared helper
• Keep mcp add guidance for its valid --force overwrite option
• Align both add and rename on explicit cortex mcp remove <name> and alternate-name guidance

Validation

• cargo fmt --package cortex-cli --check
• git diff --check
• cargo check -p cortex-cli --no-default-features --features cortex-tui
• cargo build -p cortex-cli --no-default-features --features cortex-tui --bin Cortex

Validation blockers

• cargo test -p cortex-cli mcp_cmd::handlers::tests::test_existing_server_advice_is_consistent --no-default-features --features cortex-tui is blocked before this test by a pre-existing private import error in src/cortex-cli/src/agent_cmd/tests.rs.
• Direct debug CLI execution is blocked before MCP command dispatch by a pre-existing duplicate mcp ls clap alias panic.

Addresses PlatformNetwork/bounty-challenge#53102.

[greptile-apps]

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