fix: resolve real git in test shims instead of hardcoding /usr/bin/git

[felipelalli]

The mock git shims in merge-branch-cli.test.ts and run.test.ts install a script named git into a temp dir, prepend that dir to PATH, and delegate the non-mocked commands to /usr/bin/git.

That hardcoded path is not portable, and it fails in two different ways:

• Where /usr/bin is an envfs mount — the default on NixOS with programs.nix-ld or envfs enabled — a lookup of /usr/bin/git is resolved against the caller's PATH. The caller here is the shim, whose own directory sits at the front of PATH, so /usr/bin/git resolves back to the shim and the delegation recurses without bound. On the machine this was found on, npm test never finished: the serial lane sat on run.test.ts for over an hour and the recursion had spawned ~71k processes before it was killed.
• Where /usr/bin/git is simply absent (non-FHS systems generally) the failure is quicker but still wrong — two merge-branch-cli tests fail with ENOENT from the delegating spawn.

The fix resolves the real git once, via command -v git evaluated in the test process (whose PATH has not been shadowed), and bakes that absolute path into the generated shims. The resolved path points outside /usr/bin, so the delegation can no longer re-enter the shim on any system. On FHS systems this resolves to /usr/bin/git and behaviour is unchanged.

Results on the affected machine:

• merge-branch-cli.test.ts: 13 pass / 2 fail → 15 pass
• run.test.ts: hung indefinitely → 39 pass in 8.2s

[Copilot]

Pull request overview

This PR fixes a portability and correctness issue in test-only Git shims by resolving the real git binary once (before the shim shadows PATH) and delegating to that absolute path, rather than hardcoding /usr/bin/git. This prevents infinite recursion on envfs-mounted /usr/bin setups (e.g., NixOS) and avoids ENOENT failures on non-FHS systems.

Changes:

• Add resolveRealGit() and a module-level REAL_GIT constant in two test files to capture an absolute git path from the unmodified PATH.
• Update the generated shim scripts to delegate to ${REAL_GIT} instead of /usr/bin/git.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/installer/run.test.ts	Resolve the real git once and update bash shim delegation to avoid /usr/bin/git recursion/absence.
src/cli/merge-branch-cli.test.ts	Resolve the real git once and update the Node-based shim to spawn the resolved absolute path.

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