Add AI-assistance policy, agent instructions, and code style docs

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Add three new documentation files and supporting pointer files to guide both human contributors and AI coding assistants:

• docs/Consortium-policy-on-AI.md : Project policy on AI-assisted contributions, authored by Hallberg and Ward. Establishes that a human contributor is accountable for all submitted code, requires disclosure of AI assistance in PRs, and catalogs specific risks (physical correctness, comprehension debt, vacuous tests, code churn, unsafe code). Permits AI for development assistance, debugging, documentation, CI triage, and PR review; prohibits autonomous contribution without human oversight.

• docs/AGENTS.md : Technical instructions for AI coding assistants covering the parameter system (get_param, _BUG flags, ANSWER_DATE), diagnostics (registration, posting, masking conventions), the test suite (.testing/ with tc0–tc4, test categories, verification via bitwise-identical checksums), git workflow (fork-based branching, commit message conventions with */+ prefixes, PR description style), physics domain knowledge, common development tasks (new parameterizations, diagnostics, runtime parameters, bug fixes), architecture constraints (infra/framework layering), and defensive programming practices.

• docs/Code-style.md : Fortran code style guide migrated from the wiki and extended with sections on module structure, naming conventions, memory macros (SZI_, SZIB_, etc.), the dimensional unit annotation system ([rescaled ~> MKS]), and Doxygen documentation requirements. The line-length guideline is set to 120 characters to match what trailer.py and the GitHub Action enforce.

Pointer files so AI tools auto-discover the conventions:

• CLAUDE.md (Claude Code) — uses @docs/AGENTS.md directive
• AGENTS.md (OpenAI Codex / OpenCode) — links to docs/AGENTS.md
• .github/copilot-instructions.md (GitHub Copilot) — links to docs/AGENTS.md

Also updates existing documentation:

• README.md: expanded project description, added resource links (developer wiki, forum, CVMix, TEOS-10, GOTM)
• docs/code_organization.rst: updated directory tree to reflect current layout (added FMS2, nuopc_cap, timing_tests, unit_tests; removed mct_cap; added driver descriptions)
• docs/about.rst: fixed typos
• docs/front_page.md: updated link to Code-style.md
• docs/README.md: pointed Doxygen style references to docs/Code-style.md instead of the retired wiki pages
• docs/Doxygen_* were modified to include new markdown files

And, for full disclosure, copilot/Claude Opus 4.6 was used to reconcile the redundant or conflicting information between our first draft of the docs (on google drive) and files in the repository.

PR quirks:

• This replaces and closes Add AI-assistance policy, agent instructions, and code style docs #1698 which came from a personal account and not a consortium fork.
• This also closes Add CLAUDE.md to assist development workflows with Claude Code #1691 by including the squashed commits in that PR. This is to keep the history clear of files that might not be allowed by the government.
• Also closes Update link for available diagnostics example #1697 by including the same commit from non-consortium fork.