docs: add AGENTS.md for odf and ooxml modules

[andiwand]

🤖 Generated with Claude Code

Adds design/orientation docs (AGENTS.md) for the ODF and OOXML engines, aligned with the existing oldms/ and pdf/ docs: rationale, load-bearing design decisions, and current status/gaps — not a restatement of code. Feature checklists stay in the per-module README.mds to avoid duplication.

New files

• src/odr/internal/odf/AGENTS.md — central ODF doc (all four types in one engine)
• src/odr/internal/ooxml/AGENTS.md — central OOXML doc (shared packaging / encryption / OPC relationships)
• src/odr/internal/ooxml/text/AGENTS.md — docx
• src/odr/internal/ooxml/presentation/AGENTS.md — pptx
• src/odr/internal/ooxml/spreadsheet/AGENTS.md — xlsx
• top-level AGENTS.md directory map now links the new docs

Framing

• Front-and-center for both engines: unlike oldms/pdf, ODF and OOXML keep the parsed XML DOM resident and the ElementRegistry is a thin index of pugi::xml_nodes over it — which is why ODF and docx alone can edit and save.
• Each doc's status section records foundational gaps (things worth planning next), not just missing style checkboxes.

Notable gaps surfaced from reading the code

• pptx tables are effectively broken (built with the plain builder, so table_first_column throws; table_dimensions still queries ODF table: names — copy-pasted from the ODF adapter). The same ODF-name copy-paste is latent in the docx table/cell adapter.
• pptx is_text_node copy-paste bug (checks w:t instead of a:t); full but unexposed text-edit machinery.
• Shared: agile encryption and big-endian are the big cross-format gaps; xlsx never types values or evaluates formulas; named cell-style masters are loaded but unused.

These latent-bug items are flagged as "verify and rewrite" rather than asserted as confirmed runtime failures, since they come from code reading, not execution.