[plainprogrammer]

Closes #108. Spec 027 slice C for data portability.

What it does

• New area doc docs/architecture/data-portability.md (92973c2), in the shape docs/architecture/README.md sets: scope, glossary, invariants, states, named constants, decisions not in an ADR, sources and distillation log. Every statement names its source inline. It covers the export format (specs 017, 029; ADRs 0020, 0021, 0023), the export (022), the import preview (025) and apply (028, 029), number entry's list being left out of the export (026), and the export-versus-backup guidance (030). The format's definition stays in docs/export-format/README.md, which the doc points to.
• Areas table: the data portability row now links the doc and lists the specs it covers.
• CLAUDE.md: the export, import and locations paragraphs become pointers to the area doc (spec 027 R5). A stale claim (that no spec covers the location files) was dropped; import_apply_core_spec covers them.
• Frozen specs (frozen line only, one commit each, spec 027 D4): 017 and 022 whole; 025, 026, 028, 029 and 030 in part, naming their parts in the index.
• Retired plans: 017 (d586ff6) and 022 (3d74028), with their pointers in e36505c.

Checks

• Every statement was checked against the code on main; no code–spec disagreement was found.
• The edits to shipped specs are only their Frozen: lines and retirement pointers.
• bin/ci: every step passes; 2006 examples, 0 failures, 1 pending (art_agreement_spec); also passed in the pre-push hook.

Notes

• The stored upload's retention, its sweep and the 40 MB request limits are left to the release and deploy area doc (Area doc: release and deploy #109), which is written in parallel; this doc points there.
• Area doc: release and deploy #109 edits neighbouring rows of the specs index and nearby lines of CLAUDE.md, so whichever merges second needs main merged in.
• Merge with a merge commit, and bring main in by merging, never rebasing: this branch retires plans, and their pointers cite d586ff6 and 3d74028 (docs/git-convention.md, Merging).

🤖 Generated with Claude Code