refactor: domain-model cleanup (glossary, drift-in-DB, rarity cap)

[AABur]

Summary

Domain-model cleanup driven by a grilling + domain-modeling session. Introduces CONTEXT.md (the project glossary) and aligns code, behavior, and docs with the sharpened vocabulary. Each decision came from a recorded trade-off with the maintainer.

Glossary (CONTEXT.md)

Collection, Faction vs Nation (internal code vs user-facing layer), Ability, Diff vs Drift, Reserved, Spawnable, Exile, Deck, Quantity, Baseline.

Behavior changes

• Rename attributes → abilities across internal keys, templates, and the diff. The raw JSON export keeps attributes — that key mirrors the API verbatim.
• Drop deck import — deck add is the single import path (it already does everything import did and more).
• Fix deck add quantity check — a mismatch is now only flagged when the deck needs more copies than the collection records; --update only raises quantities, never lowers them (matches the docs and the "a deck is evidence of ownership" model).
• Enforce the rarity cap on every quantity write — moved into the storage layer, so XLSX update and deck add --update now clamp to the per-rarity maximum (as the game does), with a warning. Previously only the web editor capped.
• Diff report is opt-in — sync no longer auto-writes sync-diff-*.md on apply/reject/--diff-only. The diff is shown on screen; pass --diff-report PATH to also save it.
• Drift snapshot lives in the DB, not files — a contract-drift halt no longer writes sync-schema-diff-*.md / sync-schema-observed-*.json. The observed shape is stashed in metadata; baseline accept promotes exactly the reviewed shape and clears it. CLI prints the drift; the web modal renders it. Accepting a baseline stays a deliberate CLI step.

Docs

README, CONTRIBUTING, and .gitignore updated in the same branch. "catalog" removed in favor of "Collection" / "card set".

Verification

• make check green: format + lint + mypy (47 files) + 458 tests (was 451).
• New behavior exercised end-to-end on live code, not just tests: rarity cap on the XLSX path, full drift cycle (halt → DB stash → baseline accept → clear), sync file behavior, and the web drift modal over HTTP.

Test plan

• make check
• rarity cap clamps XLSX update on an Elite card
• drift halt writes no files, stashes snapshot, baseline accept promotes and clears it
• sync writes no file by default; --diff-report PATH does
• web drift modal renders all sections and offers no apply