fix(server): distinguish the Antigravity CLI from the missing ACP runtime#10932

🟡 Medium provider/AntigravityInstallation.ts:407

On POSIX, cliOnPath fails to detect ./agy when PATH contains an empty component such as PATH=:/usr/bin, so resolution returns the generic “not installed” error instead of the CLI-specific guidance. pathCandidates filters out empty components even though POSIX treats them as the current directory; preserve them as . when building candidates.

Not changing this. The CLI check deliberately reuses pathCandidates, the same helper the provider already uses to find agy_acp_server.par on PATH and to resolve a bare-name Binary path override. Empty PATH components are dropped in every one of those lookups today, so this diagnostic is consistent with how the runtime itself is resolved.

Preserving empty components as . would change executable lookup for the whole provider and make the server search its working directory for binaries, which is outside the scope of a status-message fix. In the edge case described, the user sees the existing generic "not installed" message, which is exactly today's behavior.

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_{P2 Badge} Resolve agy using Windows PATHEXT

On Windows, when agy is available through agy.bat, a custom PATHEXT extension, or an uppercase extension in a case-sensitive PATH directory, this hardcoded pair returns false even though the shell can resolve the command. The canonical resolver already enumerates PATHEXT and upper/lowercase candidates in packages/shared/src/shell.ts:442-482; use that resolver here so these installations receive the intended CLI-specific message.

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Addressed in 70d232f. The CLI check now goes through isCommandAvailable from @t3tools/shared/shell, so Windows resolution follows PATHEXT and case variants the same way the rest of the server resolves commands, and the hardcoded extension pair is gone.

_{P1 Badge} Add focused coverage for CLI-only resolution

This branch introduces a new observable backend resolution outcome, but the commit adds no test covering CLI-only PATH detection or the resulting error detail. Add focused cases for the CLI-present and CLI-absent paths, including Windows command resolution, so this distinction cannot silently regress.

AGENTS.md reference: AGENTS.md:L109-L109

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Added in 70d232f: a focused case in AntigravityInstallation.test.ts that resolves with an empty PATH directory and gets the generic message, then with a directory holding only an executable agy and gets the CLI-specific detail. Windows command resolution is covered by the shared resolver's own tests now that this path uses it.

_{P1 Badge} Preserve missing-runtime classification in the new error

When agy is the only Antigravity executable on PATH, this detail becomes a ProviderSetupError, but isMissingInstallation in Layers/AntigravityProvider.ts only recognizes details containing not installed, missing, incomplete, or does not publish. Consequently checkProvider sets installed: true, preserves stale provider metadata, and lets onboarding/chat bypass their missing-runtime guards even though the ACP agent cannot start. Classify this error structurally or retain a recognized missing-runtime marker in the detail.

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