Probe AGENTS.md for skill routing; resolve GSTACK_ROOT in team-init

[gamerey43]

What

Two small fixes to how gstack locates itself and detects routing, plus a regression test:

1. The preamble's HAS_ROUTING probe now checks AGENTS.md as well as CLAUDE.md. Every skill's preamble runs:

   _HAS_ROUTING="no"
   for _RF in CLAUDE.md AGENTS.md; do
     if [ -f "$_RF" ] && grep -q "## Skill routing" "$_RF" 2>/dev/null; then
       _HAS_ROUTING="yes"
     fi
   done

   Non-Claude hosts (Codex, Cursor, generic agent harnesses) route skills via AGENTS.md — the de-facto cross-harness convention file. Today a repo with AGENTS.md routing but no CLAUDE.md reports HAS_ROUTING: no and gets nagged to create CLAUDE.md. This fixes that, and the routing-injection prompt stops firing for repos that already route via AGENTS.md.

2. gstack-team-init resolves GSTACK_ROOT instead of hardcoding ~/.claude/skills/gstack. Required-mode enforcement (the CLAUDE.md verification line and the generated .claude/hooks/check-gstack.sh) now resolves the install the same way skill preambles do: $GSTACK_ROOT env → ~/.codex/skills/gstack → ~/.gstack/repos/gstack → legacy ~/.claude/skills/gstack. Install instructions point at the canonical global location throughout. This stops required mode from false-blocking on installs that keep the repo at ~/.gstack/repos/gstack (the layout the setup produces on non-Claude hosts).

3. Regression test for the probe (test/routing-probe.test.ts): extracts the live generated block from review/SKILL.md and executes it across the three scenarios — both files routed, AGENTS.md-only, neither — plus a structural guard that the probe references AGENTS.md. The AGENTS.md-only case fails against the old probe, so a generator edit that breaks AGENTS.md detection is caught even when regeneration is clean.

Why

• A repo routing via AGENTS.md should be treated as routed, not prompted to add CLAUDE.md.
• Required mode should enforce against the real install layout, not a path that only exists for Claude Code installs.
• The probe's behavior is now pinned by a test, so the AGENTS.md support can't silently regress.

Testing

• bun test test/routing-probe.test.ts — 4 pass.
• Mutation check: the old CLAUDE.md-only probe returns no on an AGENTS-only repo, which the new test fails on.
• gstack-team-init required exercised in a throwaway repo: hook emits {} (allow) with the patched resolution on a global install, deny with a fake empty HOME.
• All skill docs regenerated via bun run gen:skill-docs (default + --host codex).

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