Add pre-install npm dependency guard

AGENTS.md

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+# Agent Guide
+
+This repository is a security-sensitive npm supply-chain gate. Keep changes small,
+reviewable, dependency-free, and fail-closed.
+
+## Requirements
+
+- Support Node.js 20 and newer using ESM and built-in APIs only.
+- Do not add runtime or development npm dependencies.
+- Use `node:test`; tests must not access live npm or GitHub services.
+- Inject or mock HTTP and child-process boundaries in tests.
+- Never execute dependency lifecycle scripts. Preserve `--ignore-scripts` on both
+  `npm ci` and `npm pack`.
+- Use argument-array child processes with `shell: false` and bounded timeouts.
+- Never log or report tokens, headers, environment values, registry response bodies,
+  attestation bundles, or other credentials.
+- Preserve exact repository, commit, tag, signature, integrity, and maintainer checks.
+- Do not silently trust packages that lack a normalized GitHub repository.
+- Do not commit generated evidence reports, package archives, fixtures containing
+  real project data, credentials, or tokens.
+
+## Project map
+
+- `bin/dependency-guard.js`: executable entry point.
+- `lib/cli.js`: commands and argument validation.
+- `lib/project.js`: manifest/lock parsing and pre-install checks.
+- `lib/policy.js`: policy defaults and validation.
+- `lib/verify.js`: npm metadata, provenance, GitHub commit, and tag checks.
+- `lib/check.js`: check orchestration and evidence report generation.
+- `lib/init.js`: policy generation from exact locked direct versions.
+- `lib/util.js`: URL normalization, bounded HTTP, process execution, and redaction.
+- `test/`: offline unit and orchestration tests.
+- `action.yml`: reusable Gitea composite action.
+
+## Workflow
+
+1. Inspect `git status --short` and preserve unrelated worktree changes.
+2. Read the implementation and nearest tests before editing.
+3. Add denial-path tests for malformed or adversarial inputs.
+4. Run `npm test`, then `npm run check`, then `git diff --check`.
+5. Review every spawned npm argument list and every report field before finishing.
+
+Do not commit or push unless explicitly requested.