docs: add changelog generation via git-cliff

[llbbl]

Summary

• Add CHANGELOG.md with a 0.1.0 entry covering the scaffold, conversion core, the three transports, and reverse conversion
• Add git-cliff config (cliff.toml) and changelog / changelog:preview / changelog:release package scripts
• Document the changelog workflow in the README

Changes

Documentation

• CHANGELOG.md: New file with a 0.1.0 entry covering the package scaffold, deterministic Pandoc conversion core, the three transports (local file, SharePoint, Google Drive), and docx-to-markdown reverse conversion. Dependency and API-shape notes that no commit subject can convey (fflate now a runtime dependency, google-auth-library an optional peer, PandocErrorCode/TransportErrorCode being open unions) are hand-added to the release entry.
• README.md: Document the changelog workflow and conventional commit expectations.

Build System

• cliff.toml: git-cliff configuration. Merge commits are filtered out of the changelog. A commit preprocessor normalizes the squash-merge subject of PR Add docx-to-markdown reverse conversion (Tier-2) #16, which lost its conventional prefix because GitHub uses the PR title as the commit subject -- without it that work is silently dropped from the changelog.
• package.json: Add changelog (full regeneration), changelog:preview (show unreleased without writing), and changelog:release (prepend unreleased, leaving published entries intact) scripts. changelog:release is preferred over a full regeneration once a version ships, since regenerating from history would discard the hand-added dependency/API notes above.

Test plan

• pnpm changelog:preview runs cleanly
• pnpm changelog:release prepends correctly without touching the 0.1.0 entry

package.json stays at 0.0.0 — versioning is #7's job ("version via tag"). The 0.1.0 heading in the changelog is dated 2026-07-26; if the first publish slips, re-run pnpm changelog with the real tag so the date matches the release.

Closes #19