DOC: Freshness pass on contributor- and identity-facing files

[mmcky]

Problem

Repo-meta files have drifted from reality:

• .github/copilot-instructions.md states "536 tests" and a fixed runtime; the suite is 600 tests and runtime varies by mode (~2 to ~6 minutes depending on coverage instrumentation and machine). Agents onboarding from this file inherit stale facts; prefer phrasing that ages well ("600+ tests, several minutes; never cancel").
• README.md leads install guidance with "we recommend you install the Anaconda Python distribution" — 2015-vintage advice, awkward under Anaconda's current licensing for many organisations. Lead with pip install quantecon; keep conda-forge as the conda path without Anaconda-the-distribution as a prerequisite.
• CITATION.cff is frozen at date-published: 2024-01-06 with no version: field — citation tooling emits a years-old snapshot for a package ten releases newer. Add version: and a release-checklist line to bump both per release (or wire the release workflow to do it).

Acceptance criteria

• cffconvert --validate passes; CITATION carries current version and date, and the release checklist owns keeping it true
• README install section is pip-first
• copilot-instructions facts match the suite or are phrased durably

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From the July 2026 technical-debt audit (AI-assisted; claims verified against 28d4b3b on 2026-07-25).