Align ACP agent catalog behavior

[Finesssee]

Summary

• Reworks the ACP follow-up away from static per-agent MCP metadata, since Athas does not yet have a first-party context-server source to forward.
• Merges ACP registry agents ahead of the legacy Athas marketplace catalog while preserving legacy-only agents as fallback options.
• Adds Athas agentServers settings support for custom agents and registry env/default overrides without adding a new UI surface.
• Applies configured default ACP mode/model when the agent exposes the matching session controls.
• Makes registry binary, npx, and uvx agents installable through Athas' managed tool installer so registry-only agents can be installed from the existing Agents marketplace.
• Resolves versioned Node package specs to their real package root after install, so scoped/versioned registry packages can launch from the managed wrapper.
• Sends a stable Athas user agent when loading the ACP registry and refreshes the agent selector each time it opens, so the dev UI does not stay stuck on the legacy-only cached list.
• Adds focused Rust and settings tests for catalog precedence, custom agent mapping, env/default overrides, malformed local entries, registry install mapping, Node package resolution, and settings import/normalization.

Validation

• cargo test -p athas-ai acp
• cargo test -p athas ai::acp
• cargo test -p athas-tooling resolves_
• cargo check -p athas-ai
• cargo check -p athas-tooling
• bun typecheck
• cargo +nightly fmt --all --check
• git diff --check
• Built and opened the local Athas Dev.app bundle, searched the Agents marketplace for registry-only Amp, clicked Install, and confirmed the row changed to Uninstall.
• Confirmed docs/goals/ is absent from the PR diff.
• Swept the live ACP registry for the 34 target providers the app should expose: all are present, map to a supported Athas runtime, and produce a concrete launch command.
• Verified all target provider install sources resolve: 16 binary archives are reachable, 16 npm packages resolve through npm view, and 2 Python packages resolve through pip index versions.
• cargo test -p athas acp
• cargo test -p athas acp_registry
• Started local dev with bun dev and verified the running debug binary was /Users/sw/Code/athas/target/debug/athas.
• In local Athas Dev, opened a fresh agent tab, selected a registry-backed provider (GLM Agent), sent hi, and confirmed Athas launched the provider path successfully. The provider returned its own missing Z_AI_API_KEY setup error, which verifies the UI/registry/install/launch path reached the actual ACP provider rather than failing in Athas.

[mehmetozguldev]

LGTM

[Finesssee]

Superseded by #682. The current ACP runtime/catalog branch now records this PR as an ancestor using the newer #682 implementation, so we avoid replaying the stale catalog branch and its conflicts.

[Finesssee]

Closing as superseded by #682 to keep the ACP work in one resolved runtime/catalog branch.