Assistant: surface JOIN + Work areas/SET RELATION (needs catalog work-area state)

[DDecoene]

Follow-up split out of #33.

#33 brought the Assistant catalog up to date for the post-v0.6 commands that map onto existing catalog data (CSV, SORT, SUM/AVERAGE, REINDEX, PACK). Two items were deferred because they need work-area state the catalog does not currently expose:

• JOIN — JOIN WITH <alias> TO <file> FOR <cond> [FIELDS <list>] (Add JOIN ... TO <file> to materialize a combined table from two related areas #10). A Join wizard needs to offer the user another open work area (alias), plus FOR/FIELDS inputs.
• Work areas / SET RELATION — SELECT <alias>, SET RELATION TO … INTO …, clear relation. A new "Work areas" category; the relation wizard needs the list of open areas.

Required plumbing

The catalog WebSocket message (Catalog in src/shared/types.ts) carries databases, tables, columns, indexes, reports, programs — but no list of open work areas / aliases. Add an areas list (alias, table, relation target) to the Catalog message and populate it server-side (server/Session.ts catalog handler) so the JOIN/relation wizards can offer real dropdowns.

Scope

• Add areas to the Catalog type + server catalog builder.
• Join wizard (alias picker, target table, FOR condition, optional FIELDS) → JOIN WITH … TO … FOR … [FIELDS …].
• "Work areas" category: SELECT <alias>, SET RELATION TO … INTO …, clear relation.
• Playwright e2e for each new Assistant action (per the DoD Assistant-parity step).

[DDecoene]

Moved to the v1.4.0 milestone — v1.3.0 is scoped to the lookup-columns feature (design: docs/superpowers/specs/2026-07-10-lookup-columns-design.md on release/v1.3.0).