feat: add required artifact checker helper

[ai-ag2026]

Summary

Adds a small reusable helper for checking declared required artifact paths.

Why

Agent workflows often need to verify that a worker produced concrete files rather than trusting a textual summary. This helper makes that pattern reusable without introducing a task-runtime framework. It provides the file-existence layer for the independent verification / quality-gate primitive discussed in #406.

Design

• Accepts string paths or {path: ...} declarations.
• Resolves paths through an explicit path policy.
• Verifies file existence only by default; content validation is intentionally the consumer's responsibility because content schemas are workflow-specific.
• Treats directories as not satisfying artifact declarations by default (require_file=True), with an explicit opt-out for directory artifacts.
• Returns structured JSON-serializable results.
• Does not print, mutate, or depend on any local task system.
• Lives as artifact_checks.py rather than utils/artifact_checks.py because upstream already has top-level utils.py; the dedicated module avoids import/package ambiguity while keeping the helper out of the general-purpose utils.py catch-all.

Test plan

• python -m pytest tests/test_artifact_checks.py -q -o 'addopts='
• python -m py_compile artifact_checks.py tests/test_artifact_checks.py
• git diff --check

[ai-ag2026]

CI note while this is still draft:

The red checks currently appear unrelated to this PR's two-file diff (artifact_checks.py, tests/test_artifact_checks.py):

• Windows footguns (blocking) fails on pre-existing tools/process_registry.py:588 (os.killpg(..., signal.SIGKILL)). Local diff-scoped check is clean: python scripts/check-windows-footguns.py --diff origin/main -> no footguns across 2 changed files.
• e2e fails in tests/e2e/test_platform_commands.py on /new session reset assertions. I reproduced the same e2e failure set on a detached origin/main worktree, so this is baseline breakage rather than introduced by this artifact-check helper.

Focused checks for this PR remain green:

• python -m pytest tests/test_artifact_checks.py -q -o 'addopts=' -> 18 passed
• python -m py_compile artifact_checks.py tests/test_artifact_checks.py
• git diff --check

[ai-ag2026]

Marked ready for review.

Focused PR checks remain green locally and in CI where diff-scoped checks apply. The currently red checks were triaged above as baseline/unrelated to this two-file helper PR.

[ai-ag2026]

Closing as stale/superseded by the current upstream direction.

This patch is still technically unique and applied cleanly during the latest reality check, but the original quality-gate / artifact-checker motivation has since moved upstream via the merged requesting-code-review skill consolidation (#4854, absorbing #4459 and closing #406). A standalone core required artifact checker helper no longer matches the current upstream shape unless maintainers explicitly want a reusable core API later.