fix: enforce required gstack across Claude Code and Copilot

[time-attack]

A project could say gstack was required but Copilot could still open a skill when gstack was missing. The official main branch reproduces this today.

Now Claude Code and Copilot stop the skill and explain how to install gstack. When gstack is installed, the skill opens normally.

What changed

• Replace the shell-specific check with one cross-platform check.
• Handle Claude Code’s Skill name and Copilot’s lowercase skill name.
• Return the denial format understood by both runners.
• Stop safely when the installation or project directory cannot be checked.
• Migrate projects away from the old shell-specific check.
• Explain the behavior in plain English in the README.

Real-runner verification

Using Copilot CLI 1.0.70 with gstack deliberately absent:

• Official main: the protected skill loaded and returned RUNNER_SKILL_EXECUTED.
• This branch: the skill returned success: false, code: denied, and never executed.

Both CLI processes exited 0, so verification used the actual tool result and execution token rather than the process exit code.

Live repository gate

The identical probe failed on official main and passed with this branch:

• miru-web: fail → pass
• gbrain: fail → pass
• zotero-arxiv-daily: fail → pass
• plane: fail → pass
• twenty: fail → pass

Secondary checks

• Required-team tests: 33 passed, 0 failed
• Shell syntax: passed
• Diff check: passed
• Windows-safe suite reached one unrelated browser terminal test failure. The exact test fails identically on official main and this branch: 18 passed, 1 failed.

No version or release files changed.

This directly supersedes #2217 with the combined fix and real-runner verification.

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