Add ODT output format

[rapatao]

What

Add odt as a supported output format (-f odt), following the pattern
documented in CONTRIBUTING.md / README.md "Adding a new output format".

Why

OpenDocument Text is the native format for LibreOffice/OpenOffice, and
required in some EU/government contexts. Overlaps with DOCX (#8) as an
"editable office doc" target — niche relative to DOCX's Word audience, but
same zip-of-XML approach, no external dependency needed (pure Go).

Approach

1. New package internal/converter/odt/odt.go.
2. Implement converter.Converter, mapping the goldmark AST (headings,
   paragraphs, lists, code blocks, tables, bold/italic/links) to
   content.xml + supporting parts (mimetype, META-INF/manifest.xml,
   styles.xml, meta.xml).
3. init() calls converter.Register("odt", Converter{}).
4. Blank-import the package in main.go.
5. Tests under internal/converter/odt/ — at minimum, assert the output is
   a valid zip with expected parts, spot-check rendered text content.
6. Add odt to the formats table in README.md.

Note: mimetype file must be the first zip entry, stored uncompressed
(ODF spec requirement) — differs from DOCX's zip layout, watch for this.

No CLI changes needed — -f odt works automatically once registered.