[REFACTOR] Move gh_issues.py into vig-utils and add entry point

[c-vigo]

Description

Move scripts/gh_issues.py into packages/vig-utils, create an entry point for running it, remove the shipped asset copy that is no longer needed, and adapt all references to the new location.

Files / Modules in Scope

• scripts/gh_issues.py (source to migrate)
• packages/vig-utils/** (target module + entry point wiring)
• assets/workspace/.devcontainer/scripts/gh_issues.py (remove shipped asset copy)
• References/invocations that currently point to old paths

Out of Scope

• New gh-issues features or UI changes
• Functional behavior changes beyond path/entrypoint migration
• Broader rewrite work tracked in #145

Invariants / Constraints

• just gh-issues continues to work end-to-end
• Behavior/output remains equivalent after migration
• Existing tests and CI checks pass

Acceptance Criteria

• scripts/gh_issues.py is moved under packages/vig-utils
• A supported entry point is created for invoking gh-issues
• assets/workspace/.devcontainer/scripts/gh_issues.py is removed
• All references are updated to the new entry point/location
• just gh-issues works after the migration
• All relevant tests pass
• TDD compliance (see .cursor/rules/tdd.mdc)

Changelog Category

Changed

Additional Context

Related to parent issue #145.

[c-vigo]

Subsumed by #217: [REFACTOR] Reorganize scripts/ — project-specific vs shared vig-utils. Tracking migration work there to keep script reorganization in one place.