repo hygiene: make Copilot primary executor in agent instructions

[mdheller]

Closed by PR #141.

Resolution:

• Updated AGENTS.md.
• Updated .github/copilot-instructions.md.
• Clarified Copilot-primary issue-first bounded PR work.
• Clarified that comments alone are not delivery artifacts; GitHub PR/branch/commit/merge is required.
• Preserved docs-only instruction scope.

Validation evidence is recorded in PR #141.

[mdheller]

@copilot Follow-up: please open one bounded PR for issue #137. The full scope and validation commands are in the issue body. Keep this docs-only: update AGENTS.md and .github/copilot-instructions.md; do not modify workstation implementation, workflows, manifests, or runtime behavior.