Respect CORTEX_HOME for ACP agent lookup

[GreyforgeLabs]

Summary

• Resolve ACP agent lookup through Cortex's existing home resolver instead of hardcoding ~/.cortex.
• Keep the current early --agent validation behavior while honoring CORTEX_HOME.
• Add a binary integration regression that creates an agent under a temporary CORTEX_HOME and verifies cortex acp --agent does not fail with Agent not found.

Fixes PlatformNetwork/bounty-challenge#51939.

Validation

• cargo test -p cortex-cli --test acp_cortex_home --no-default-features --features cortex-tui -- --nocapture
• cargo check -p cortex-cli --no-default-features --features cortex-tui
• cargo fmt --package cortex-cli --check
• git diff --check

Note: cargo test -p cortex-cli acp_cmd::tests::test_acp_agent_lookup_uses_cortex_home -- --nocapture was attempted first and hit the existing default-feature alsa.pc system dependency in this environment before the test was moved to the binary integration suite.

[greptile-apps]

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