Add offline e2e script checks

[okJiang]

Summary

• Add make e2e-check for offline validation of e2e scripts.
• Add a PR/push/manual GitHub Actions workflow that runs the guardrail without requiring dev/staging, secrets, or network access.
• Document the offline guardrail in e2e/README.md.

Details

The new guardrail checks shell syntax, set -euo pipefail, Python syntax via compile(...), and the v1alpha2 wrapper script targets. It intentionally does not call any Mem9 API or live backend.

Validation

• make e2e-check
• make test-cover

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Name	Link
🔨 Latest commit	f7ff3f7

[okJiang]

LGTM. Minimal offline guardrail — well-scoped first tier.

[okJiang]

Closing this PR based on the review discussion. The standalone offline e2e script guardrail is too narrow to justify an additional CI workflow, and it does not validate Mem9 API behavior. We will keep the current automation focus on the local server smoke coverage in #263, which runs a real mnemo-server with local pgvector and validates API behavior without dev/staging dependencies.