Harden ACP agent install metadata

[Finesssee]

Summary

• Validate ACP managed-install metadata before invoking the shared tool installer
• Return clearer errors for unsupported installs, missing runtime packages, and binary installs without download URLs
• Add focused ACP install metadata and wrapper command tests

Validation

• cargo test -p athas ai::acp
• cargo fmt -p athas --check
• cargo check -p athas
• git diff --check

Note: bun check:rust currently fails on existing workspace formatting drift outside this PR (crates/runtime/src/lib.rs, crates/tooling/src/registry.rs, crates/tooling/src/types.rs). This PR leaves those files untouched.

[Finesssee]

Closing this for now to keep the ACP catalog/install work separated cleanly. This install hardening is still useful, but it touches the same ACP command module as the open catalog PR and would create avoidable review overlap/rebase churn. I’ll bring it back as a fresh follow-up after the catalog PR lands.