build: switch to pnpm and split cspell configuration

[frankbuckley]

Note

Drafted by a LLM-based AI tool (Codex/GPT-6).

Switch Markdown tooling from npm to pnpm 12.10.1. Import the npm lockfile into pnpm-lock.yaml, preserve all 109 dependency versions and integrity hashes, and remove package-lock.json. Pin pnpm in package.json, set up pnpm and its cache in CI, and install with --frozen-lockfile.

Replace cspell.json with cspell.yaml. Move all 47 custom words into cspell-words.txt, one word per line, and enable that file as a project dictionary. Preserve the British English setting and ignored paths.

Update the README and all three agent instruction files to use pnpm. Document where to add custom spelling terms. Mark the generated lockfile for collapsed GitHub diffs.

Validation on Node.js 24.21.0 and pnpm 12.10.1:

• Frozen installation passes without changing the lockfile and rejects a changed dependency version.
• Spellcheck, Markdown formatting and Prettier checks for the changed JSON and YAML files pass.
• All imported dependency versions and integrity hashes match the npm lockfile; the three instruction files match except for their first heading.
• All 47 dictionary words match the previous config in spelling, case and order. Cspell loads every word from the new file and still flags an unknown word.
• Actionlint passes after excluding its warning about the existing ubuntu-26.04-arm runner label, which its current version does not recognise.
• git diff --cached --check passes.