stash schema build: port the v2 capability column picker to EQL v3 domains #751

Prerequisite for #707 (EQL v2 removal). Port stash schema build's v2 capability-based column picker to EQL v3 fixed domains.

Why this is a v2-removal prerequisite

Like #693 (ALTER COLUMN) and #657 (DynamoDB), this is an unfinished v3 feature, not a removal side-effect — v2's presence currently disguises it as "the picker works." It needs building whether or not v2 is ever deleted. It is called out in docs/plans/2026-07-20-eql-v2-removal-scope.md §3.4 and §9 (recommended order, item 4) but — unlike the other two prerequisites — has no tracking issue.

What the picker is

stash schema build drives the full v2 chainable capability model and reaches production:

• packages/cli/src/commands/init/types.ts:9 — export type SearchOp = 'equality' | 'orderAndRange' | 'freeTextSearch'
• packages/cli/src/commands/init/lib/introspect.ts:102 — allSearchOps() produces SearchOp[]
• .../introspect.ts:119 — selectTableColumns(), with the p.multiselect column picker at :147 and the searchable flow at :175
• .../introspect.ts:197 — buildSchemasFromDatabase()
• packages/cli/src/commands/schema/build.ts:5,58,65 — schema build calls buildSchemasFromDatabase() then generateClientFromSchemas()

Common misread (already burned us once — scope §3.4)

The init path is already v3-neutral: it sets schemas: [] (init/steps/build-schema.ts:85,147) and only calls generatePlaceholderClient(). Tracing init alone concludes SearchOp/ColumnDef are dead — they are not. stash schema build still drives the full capability picker. Confirm against the schema build path, not init.

The actual work

The capability tuple (equality | orderAndRange | freeTextSearch) has no v3 analogue — v3's query capability is fixed by the concrete domain type (types.TextSearch, types.IntegerOrd, types.Json, …), not by chainable toggles. So the port is a redesign of the picker UX from "toggle a set of capabilities per column" to "choose a domain per column," plus regenerating v3 client output. This is design work, not a mechanical rename.

Open design questions:

• How to map an introspected Postgres column type to the candidate v3 domain(s) offered.
• Whether the picker offers a single domain choice or still exposes a capability-like grouping mapped onto domains.
• v3 client codegen from the domain choice (the generators in init/utils.ts are still v2 — see scope §4/§7 generatePlaceholderClient).

Distinct from

• stash init: column picker has three usability/safety issues #396 — init column-picker usability/safety, not the v3 port.
• Adopt EQL V3 self-configuring domain types in @cipherstash/stack schema #534 — v3 self-configuring domains in the @cipherstash/stack schema builder, a different layer from the CLI picker.

Cross-link both so they aren't conflated.

Done when

• stash schema build produces v3 schemas/domains with no reference to SearchOp/allSearchOps.
• SearchOp / ColumnDef and the v2 generators are removable without breaking schema build.
• Unblocks the EQL v2 removal (Remove EQL v2 repo-wide (umbrella) #707).