fix: hide literal strikethrough markers for empty cancelled exports

[vycdev2]

Summary

• Export empty cancelled tasks without showing literal ~~~~ markers in Markdown viewers.
• Add a Markdown rendering regression and note the user-visible fix in the changelog.

Verification

• ./node_modules/.bin/mocha -r ts-node/register test/markdown-export.spec.ts --grep 'empty cancelled task' — failed before the fix (literal ~~~~).
• npm test — passed, 416 tests.
• npm run compile — passed.
• npm run typecheck — passed.
• npm run format:check — passed.
• git diff --check — passed.

Risk

• Low: only empty cancelled task output changes; nonempty tasks keep existing Markdown syntax. The empty state uses <s></s> instead of nonrendering ~~~~, so viewers that disable inline HTML will display the literal tags.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.