Add local file transport and CI

[llbbl]

Summary

• Add a shared Transport contract with typed transport errors and a local file transport implementation.
• Add deterministic, idempotent local storage with opaque canonical IDs, safe custom mapping, lexical cross-platform path validation, preserved causes, and documented symlink limits.
• Add GitHub Actions CI for Node 20/22/24 with pinned Pandoc 3.10.1 integration coverage.

Notes

• Tests and README documentation are included.
• pnpm is pinned to 10.34.5 because pnpm 11.9 requires Node >=22.13 while this package supports Node >=20.
• Release publishing and optional Docker remain on CI + npm publish #7.

Test plan

• Exact pnpm 10.34.5 frozen install
• pnpm format:check
• pnpm check
• Forced Pandoc test run: 60 passed, 1 skipped
• Node 20.20, 22.22, and 24.13
• pnpm pack --dry-run
• actionlint
• Independent reviewer PASS

Closes #3
Refs #7