fix: authenticate glab and agent CLIs through the login shell environment

[AbdelrahmanHafez]

Follow-up to #144, which fixed gh authentication for GUI launches and left glab out of scope. Same story: a Dock-launched Codiff inherits launchd's minimal environment, the single-instance lock keeps it for the lifetime of the process, and glab never sees a GITLAB_TOKEN that only exists as a login shell export. GitLab requests fail with:

To get started with GitLab CLI, please run: glab auth login

While auditing the other spawn sites for the same gap, I found the agent CLIs have it too: claude, codex (exec and app-server), opencode (run and serve) and pi all spawn with plain process.env, so keys like ANTHROPIC_API_KEY or OPENAI_API_KEY that only live in the user's shell profile never reach them from a GUI launch.

This PR:

• Adds getCommandEnvironment to login-shell-environment.cjs: { ...(await getLoginShellEnvironment()), ...process.env }, so the login shell only fills variables the process doesn't already have
• Spawns glab with it in glabApi, mirroring the gh fix
• Spawns the six agent CLI sites with it, keeping OPENCODE_PERMISSION as an explicit override on top for OpenCode
• Routes runGhApi through the same helper so the precedence rule is defined once
• Keeps the agent suites hermetic with a beforeEach that clears SHELL, since every runner now resolves the login shell; the new tests bring their own fake login shell

The commits are ordered test first, then fix, for both halves. The glab regression test fails on the exact error above before the implementation, and the agent tests assert the spawned environment through the existing command transport capture, including that the process environment wins over the login shell.

I looked at the remaining spawn sites and left them alone on purpose: opencode export reads local session files, cloudflared authenticates through a browser flow and an on-disk cert, and the git spawns work fine from GUI launches today.

Created with Fable 5 xhigh, reviewed by Sol 5.6 xhigh.

[cpojer]

Nice!