fix(agent-guard): make the fresh-clone hook guard cross-platform (Windows)

[milamberspace]

Summary

Follow-up to #786 (merged): that PR guarded the committed agent-guard hook command with a POSIX shell conditional — [ -f … ] && python3 … || true — so the hook no-ops on a fresh clone before /magpie-setup has synced agent-guard.py in. But that form only works in a POSIX shell; on Windows (PowerShell, no Git Bash) the committed hook breaks.

Replace it with an inline python3 -c existence check that is cross-platform and needs no shell conditional:

python3 -c "import os,sys,subprocess; p=os.path.join(os.environ.get('CLAUDE_PROJECT_DIR',''),'.claude','hooks','agent-guard.py'); sys.exit(subprocess.call([sys.executable,p]) if os.path.isfile(p) else 0)"

• no-ops (exit 0) when the script is absent (fresh clone);
• runs the guard and propagates its exit code (so a deny still blocks) when present;
• reads CLAUDE_PROJECT_DIR from the environment in Python, so it doesn't depend on $VAR vs %VAR% shell expansion.

Updated in the two places that ship the snippet: skills/setup/adopt.md and tools/agent-guard/README.md.

Found by an adopter (Apache JMeter) reviewing the same change downstream — the reviewer asked "Does this work with windows?" on the adoption PR.

Type of change

• Bug fix (docs / committed template)

Test plan

Doc / template change. The guarded command is valid on both POSIX and Windows (pure python3 -c, no shell conditional); it no-ops when the script is missing and runs + propagates the guard's exit code when present.

[milamberspace]

Heads-up for maintainers: this cross-platform fix may be superseded by #824.

That issue proposes writing the agent-guard hook into the gitignored, per-machine .claude/settings.local.json at setup (alongside the script) rather than committing it to .claude/settings.json. If that lands, the wiring only ever exists once the script is present, so no fresh-clone guard is needed at all — and the command reduces to a plain python3 ".../agent-guard.py", which is already cross-platform. In other words #824 would make both the #786 guard and this python3 -c rewrite unnecessary.

Happy to close this in favour of #824, or keep it as the interim fix if you would rather ship the smaller change first — your call.

[potiuk]

Oh... Windows support :)