Add shared AI instructions and update Python support

[anth-volk]

Fixes #1748
Fixes #1838

Why

Repository guidance currently lives primarily in CLAUDE.md, which makes durable engineering instructions difficult to share consistently across Codex, Claude, Copilot, and other AI coding tools. The package also still declared Python 3.9 and 3.10 compatibility from the earlier Census compatibility effort even though PolicyEngine US subsequently removed support for those versions.

Summary

• Add AGENTS.md as the shared AI contributor-instruction entry point.
• Convert CLAUDE.md and add Copilot instructions as thin adapters into common documentation.
• Add task-specific engineering guidance for pull requests, testing, model structure, dataset sources, and documentation review under docs/engineering/skills/.
• Link the AI instructions to the repository and organisation contribution guides.
• Require Python 3.11 or newer in package metadata and uv.lock.
• Remove Python 3.9/3.10 classifiers, dependency rules, compatibility CI jobs, and source adaptations.
• Set Ruff compatibility to Python 3.11 and document the supported range as Python 3.11–3.14.
• Add Towncrier fragments for both tracked changes.

Verification

• Rebased onto current main (672e6d32248107590ccd77fc367cfeb087465a66).
• uv run --extra dev ruff format . — 999 files unchanged.
• uv run --extra dev ruff check . — all checks passed.
• uv lock --check — passed.
• uv build — source distribution and wheel built successfully.
• Built wheel metadata contains Requires-Python: >=3.11, only Python 3.11–3.14 classifiers, and tables>=3.10.2.
• Focused dataset-source, simulation-source, and build-metadata tests — 16 passed, one optional-dependency test skipped.
• git diff --check — passed.

make format could not run because ruff was not directly available on the shell path, so its two commands were run through the locked development environment. The full test suite and documentation build were not run locally; the pull-request workflows will run both.