Stop the gate stamping VCS metadata into throwaway binaries

[charlesgreen]

Work item 3 from #119, promoted from "consider" to "the fix" by evidence from two agent runs.

The failure

Every agent run reports a red gate:

error obtaining VCS status: exit status 128
Use -buildvcs=false to disable VCS stamping.

It fails at go build ./... before any repo code compiles, so the gate says red for a reason unrelated to the change under test. On #115 the agent worked around it with TMPDIR=$PWD/.tmp go test ./cmd/simplycubed, which passed — the code was fine, the environment was not.

A gate that cannot run where the loop runs is not a gate. One that fails for unrelated reasons is worse, because it invites someone to weaken it to get green.

Why this is the right fix rather than a workaround

The gate builds binaries and throws them away. Stamping git metadata into them buys nothing and couples the gate to git plumbing it has no reason to touch. Go's own error message recommends exactly this flag.

Nothing user-visible depends on it:

• internal/buildinfo reads the module version from debug.ReadBuildInfo, not the VCS stamp
• release binaries are built separately in .github/workflows/release.yml with the version passed via -ldflags -X

Honesty about reproduction

I could not reproduce the failure on a developer machine or in a runner probe (30731396430) — go build with stamping on succeeded in a worktree under /tmp on a runner. The agent reproduces it consistently, twice.

So this is not "I found the root cause and fixed it." It is "the gate has no reason to do the thing that is failing, and Go's own error names this flag as the remedy." The remaining question of why it differs is still #119.

Verified: gate passes normally, and from a worktree with TMPDIR inside it.

[codecov]

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