Replace deprecated Gemini CLI provider runtime with AGY

[gcp007-ops]

Problem

The current google-gemini-cli provider slot still represents the old Gemini CLI workflow, but the gemini binary is deprecated/unsupported in practice. Keeping a fallback to gemini does not help because that path no longer works at runtime.

Expected direction

Route the existing Gemini CLI provider slot through Google Antigravity CLI (agy) instead of the deprecated gemini binary, while preserving compatibility for existing Nexus settings.

Proposed behavior

• Keep the provider id google-gemini-cli so existing settings and saved provider selections continue to load.
• Relabel user-facing surfaces to Antigravity CLI / Google (Antigravity), rather than Gemini CLI.
• Use AGY for runtime execution and local auth checks.
• Manage the AGY MCP entry through ~/.gemini/config/mcp_config.json.
• Reject malformed or blank AGY MCP config files instead of silently overwriting them.
• Normalize legacy Gemini CLI model selections to the current AGY-backed model labels.
• Do not keep a runtime fallback to the old gemini CLI, since that path is no longer functional.

Downstream status

I have a local patch on top of Nexus 5.12.1 that implements this direction, with focused tests and a successful build. I can open a PR if this approach matches the maintainer direction.

[gcp007-ops]

Additional downstream finding after local testing: AGY print mode can keep emitting planner progress without a final ModifiedResponse, which leaves Nexus chat looking like it is processing indefinitely until the CLI exits.

I updated the local patch to make this bounded:

• --print-timeout reduced from 5m to 60s
• local process watchdog added at 75s
• AGY timeout output / watchdog timeout maps to LLMProviderError code PROVIDER_TIMEOUT

Local commit: a872f8353df901a64e9e5927b3ee8eac9be70ded.

[gcp007-ops]

Additional downstream finding from local testing:

AGY print mode can terminate without a useful final stdout payload after repeated planner/tool activity, even when the process itself exits. In Nexus this exposed a second UI/state issue: if a provider error or empty complete response happens before the first streamed token, the assistant placeholder can remain isLoading: true, making the chat appear to process forever.

Local patch update on the 5.12.1 patch branch:

• Treat empty successful AGY stdout as PROVIDER_TIMEOUT with a visible message instead of a successful empty assistant response.
• Mark assistant placeholders state: invalid / isLoading: false when generation fails before any token streams.
• Ensure complete-but-empty stream results also clear isLoading.

Local commit: 066cd4e972059786df6f9cdd37540989a01d1b3f (fix: clear AGY empty-response chat loading).

[ProfSynapse]

Hey so I implemented this in the next release but honestly it's a bit useless...it can't use tools. There is no like JSON/Tool mode output so it's text only for now until they get their shit together...I don't want to give it full range of it's coding tools. Needs to filter through the nexus MCP and right now there is no reliable way to do that.

I don't want to use ~/.gemini/config/mcp_config.json because that changes the global setting and again does not limit all the other coding tools it can use.