docs(dev): rewrite ARCHITECTURE and TESTING as the current design, trim PLAN, and replace copilot-instructions with AGENTS.md

[quantecon-services]

PR 5 of the user manual (#178). It runs alongside PRs 2–4. The rule is from #178, decision 4: docs/dev describes the project as it is, with no dated status blocks, "corrected on" notes or resolved questions.

• docs/dev/ARCHITECTURE.md: rewrite as the current design. Drop "Open Questions (Resolved)", "Success Criteria", "Migration Path" and the stale performance targets.
• docs/dev/TESTING.md: rewrite as how testing works now (harness, fixtures, canary). Drop the dated status, phases, rollout plan and success metrics, including the stale "~10-20 sec" at :188.
• docs/dev/PLAN.md: trim to current priorities, consumers, rollout state and dependency policy.
• Replace .github/copilot-instructions.md with a root AGENTS.md that points at docs/dev. The stale lines at :133 ("Updated: June 16, 2026") and :137 (the lecture-dp "Next" line), left over from docs: sweep to the Anaconda 2026.06 baseline and measured container sizes #106, go with it.