Add CHANGELOG.md before first npm publish #19

Follow-up from #6 (PR #16). Pre-publish gate for #7.

The package is still 0.0.0 and nothing has shipped, so this gates the first npm publish rather than any single change.

The strongest reason is not an API addition — it is that fflate moved from devDependencies to dependencies in #6, because DOCX archive inspection now runs at runtime. That changes what every consumer installs and is supply-chain-visible, so it belongs in a changelog entry.

Also worth an entry in the first release:

• PandocErrorCode widened from 8 to 12+ members across GoogleDriveTransport (extract from polydoc) #5 and Reverse conversion: docx → clean md (Tier 2) #6. Source-compatible for consumers narrowing on it; breaks only an exhaustive switch with a never assertion.
• google-auth-library became an optional peer dependency in GoogleDriveTransport (extract from polydoc) #5.
• Reverse conversion (convertDocxToMarkdown) is the package's second public API surface alongside the forward conversion and the transports.

Tasks

• Add CHANGELOG.md with a 0.1.0 entry covering the scaffold, conversion core, the three transports, and reverse conversion.
• Decide whether to adopt git-cliff / conventional-commit-driven generation (commits already follow conventional commits) or hand-write entries.
• Wire the changelog into the release workflow from CI + npm publish #7 so a tagged release and its entry cannot drift apart.