Add APA DOCX manuscript export with reference deduplication

[matrixflora]

Summary

Checklist

• Tests pass locally: uv run pytest
• Linting passes: uv run ruff check src/ tests/
• Type check passes: uv run mypy src/
• Changelog / ROADMAP updated if this is a user-facing change
• Docs updated if the public API surface changed (CLI flags, REST endpoints, MCP tools, config keys)
• New dependencies reviewed for license compatibility (Apache 2.0-compatible only)
• No unrelated cleanup mixed in (keep diffs focused)
• CLA signed if this is an external contribution (see CONTRIBUTING.md)

Test plan

Related issues

[lfnothias]

Blocking review — not mergeable as-is, and it conflicts with the "stay generalizable" goal:

1. Debug prints left in: print("OUTPUT PATH:", output_path) in both _generate_answer and _generate_single_paper_answer.
2. Always-on side effect: a .docx is written to output/{id}_manuscript.docx on every agentic answer. This should be opt-in (a config flag, default off), not forced on all users.
3. Core dependency for a niche feature: python-docx is added to base dependencies. It should be an optional extra (e.g. [docx]), imported lazily so the dep is only needed when the feature is enabled.
4. Likely runtime bug: export_apa_docx/to_apa treat papers as dicts (paper.get(...)), but the orchestrator passes Paper objects — the bare except would swallow the resulting AttributeError and silently write blank references.
5. No test.

Happy to rework this into an opt-in, optional-extra, tested exporter (matching the pattern used elsewhere). Converting to draft until then.

[lfnothias]

Superseded by #24, which reworks this into a safe, generalizable form: opt-in (off by default) via config.rag_modes.agentic.export_apa_docx, python-docx as an optional [docx] extra (lazy import), tolerant of Paper/Author objects (this PR would have silently produced blank refs on real objects), no debug prints, export failures isolated from the answer path, and hermetic tests. Closing in favour of #24.