docs(claude): prune AI instructions to principles + conservative-maintainer rules

[sbryngelson]

Description

Prunes the Claude Code instruction files from ~3,300 words across 5 files to ~1,600 words across 2 files, shifting from exhaustive inventories toward principles backed by concrete reasons.

What changed

• CLAUDE.md — new "How to Work Here" section framing the repo's three load-bearing constraints (four CI-gated compilers, silent failure modes, toolchain-first), plus conservative-maintainer rules against over-engineering: smallest correct change, diff discipline, no-bloat list, test justification criteria, and a before/after-editing protocol with a ~100-net-line stop rule.
• .claude/rules/common-pitfalls.md — now holds only the silent-failure traps documented nowhere else (buff_size routing, eqn_idx, GPU_LOOP silent-serial on Cray/AMD, CASE_OPT_PARAMS rebuild trap, QPVF_IDX_VARS mismatch, checker placement, golden-file discipline).
• Deleted fortran-conventions.md, parameter-system.md, gpu-and-mpi.md — their content either duplicated the lint-gated user docs (contributing.md, gpuParallelization.md, testing.md, case.md) and is replaced by pointers, or was a unique gotcha and moved into common-pitfalls.md.

Rationale

• Capable models follow project-specific facts and gotchas far better than restated general philosophy; what they don't need is a second, unchecked copy of content the lint-gated docs already maintain. Most facts now live in exactly one place, and the bulk of them in docs that lint_docs.py freshness-checks on every precheck/CI run — making the instructions much harder to go stale.
• The .github/workflows/claude-code-review.yml glob (.claude/rules/*.md) needs no change.

Type of change

• Documentation update

How has this been tested?

• ./mfc.sh precheck (all 7 checks pass, including doc-reference and spell checks; runs via pre-commit hook)

Checklist

• I ran ./mfc.sh format before committing my code
• This PR does not introduce any repeated code

[Copilot]

Pull request overview

This PR restructures the repository’s Claude instruction set by consolidating “how to work here” guidance into CLAUDE.md and moving truly silent-failure “gotchas” into a single .claude/rules/common-pitfalls.md, while deleting duplicated instruction files that overlap with CI/lint-gated documentation.

Changes:

• Replaces “Working Style” with a more principle-driven “How to Work Here” section and refreshes command/workflow guidance in CLAUDE.md.
• Refactors .claude/rules/common-pitfalls.md to focus on silent-failure traps (indexing, GPU macro pitfalls, parameter gotchas, golden-file discipline).
• Deletes three .claude/rules/*.md files whose content is now either moved into common-pitfalls.md or replaced by pointers to docs/documentation/*.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
CLAUDE.md	Consolidates maintainer-mode workflow principles, updated command guidance, and pointers to lint-enforced sources of truth.
.claude/rules/common-pitfalls.md	Centralizes the “silent failure” traps (indexing, GPU macro usage, parameters, tests) into one short reference.
.claude/rules/parameter-system.md	Deleted; prior content is replaced by pointers and selected gotchas moved into common-pitfalls.md.
.claude/rules/gpu-and-mpi.md	Deleted; GPU/MPI guidance is now primarily referenced via docs/documentation/gpuParallelization.md plus key silent-failure warnings in common-pitfalls.md.
.claude/rules/fortran-conventions.md	Deleted; forbidden-pattern details now point to toolchain/mfc/lint_source.py and lint-gated documentation.