Clarify agent list hidden counts

[GreyforgeLabs]

Summary

• split agent list filtering into a matching population and visible rows
• count hidden agents within the same filtered population used for role totals
• update the footer to show Showing visible of total agents when hidden agents are omitted

Fixes PlatformNetwork/bounty-challenge#53156

Tests

• cargo fmt --package cortex-cli --check
• git diff --check
• cargo check -p cortex-cli --no-default-features --features cortex-tui
• cargo run -p cortex-cli --no-default-features --features cortex-tui --bin Cortex -- agent list | tail -n 4
• cargo run -p cortex-cli --no-default-features --features cortex-tui --bin Cortex -- agent list --all | tail -n 4

[greptile-apps]

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