Revert commit SHA resolution to git merge-base

[Copilot]

The footer's commit link was broken because PR #601 changed getCommit() to use GITHUB_SHA / git rev-parse HEAD, both of which can produce synthetic merge commit SHAs that don't exist in the public repo history.

Changes

• src/frontend/scripts/write-git-env.mjs: Reverts getCommit() to git merge-base origin/main HEAD with upstream/main HEAD as a fork fallback — this always resolves to a real commit on main.

// Before (broken): resolves GITHUB_SHA, which is a synthetic merge commit in CI
return getExplicitCommit() ?? tryExec('git rev-parse HEAD') ?? '';

// After (fixed): resolves the actual ancestor commit on main
return (
  tryExec('git merge-base origin/main HEAD') ??
  tryExec('git merge-base upstream/main HEAD') ??
  ''
);

The non-breaking .env.local file I/O refactor from PR #601 is retained.

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[Copilot]

Pull request overview

This PR fixes the frontend footer’s commit SHA link by changing how the build determines the commit: instead of using CI-provided SHA values that may refer to synthetic merge commits, it resolves a commit that exists on the public main history via git merge-base.

Changes:

• Remove CI/env-based SHA resolution and compute the commit via git merge-base .../main HEAD.
• Add upstream/main as a fallback remote for fork workflows while keeping the .env.local write logic from #601.

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