fix: detect externally installed ACP agents

[H1d3rOne]

Summary

• broaden ACP agent detection beyond Codeg-managed install records
• detect npm/NPX agents from global package metadata, npm prefix shims, and --version output, using unknown when launchable but version is not provable
• distinguish upstream Claude/Codex CLIs from required ACP adapters in Settings and connect-block messaging
• allow system binary fallbacks for binary agents, including OpenCode CLI packaged at macOS app-bundle paths
• align uvx/Hermes readiness with system uvx / system hermes fallbacks so Settings, preflight, and connect gating agree

Verification

• cargo check
• cargo clippy --all-targets --features test-utils -- -D warnings
• cargo test --features test-utils commands::acp::tests::
• pnpm eslint .
• pnpm vitest run src/components/settings/acp-agent-settings.test.tsx
• git diff --check