feat: filesystem adapters via markup-generator (issue #25)

[atherdon]

Implements issue #25 PR A + list adapter.

generate-template

• src/adapters/list-files.ts — directory scan via markup-generator.listTemplateFiles
• src/adapters/payload-files.ts — dataPath / {slug}.data.js via loadData + pathExists
• Default payload path no longer existsSync(dataDir). Missing payload → NO_PAYLOAD unless payload, dataPath, samplePayloads, loadPayload, or useDataFiles: true.
• generator.ts no longer imports node:fs

Depends on

LLazyEmail/markup-generator#275 (pathExists, listTemplateFiles). Merge and publish that first, then bump markup-generator here.

[atherdon]

Updated to markup-generator ^3.2.0 (merged #275: pathExists, listTemplateFiles).

• package.json dependency bumped from ^3.1.5 → ^3.2.0
• Re-exported pathExists, listTemplateFiles, loadData from this package root

Publish markup-generator@3.2.0 to npm if CI cannot resolve the version, then re-run.