fabric: consider proxyVendor=true for stable vendorHash across Go toolchain updates

[JacobPEvans-personal]

Context

In #780, the fabric vendorHash had to be recomputed when nix-darwin bumped its nixpkgs (Go toolchain skew between nix-ai's pinned nixpkgs and nix-darwin's).

Gemini code-assist suggested setting proxyVendor = true in modules/fabric/package.nix:

When proxyVendor is false (default), Nix runs go mod vendor to generate the dependency tree. Output is sensitive to Go toolchain version (e.g., modules.txt formatting changes). Enabling proxyVendor makes the build fetch modules directly from the Go proxy — more stable, platform-independent vendorHash.

Action

1. Set proxyVendor = true in modules/fabric/package.nix
2. Recompute vendorHash (will be different from current)
3. Verify build still passes on both nix-ai CI and nix-darwin

Why deferred

PR #780 was scoped to unblock nix-darwin's flake-update urgently. This is a follow-up robustness improvement.

Assisted-by: Claude noreply@anthropic.com

[JacobPEvans-personal]

🤖 Issue Solver attempted this issue and stopped.

Reason: fix requires running nix build to recompute vendorHash — not possible in the cloud sandbox
Phase reached: triage

Human review needed. The agent will not retry this issue for 7 days.