docs: update PostGIS skill with withinDistance, measurements, transformations, aggregates

[pyramation]

Summary

Updates search-postgis.md to document new PostGIS features added in constructive-io/constructive#864. Documentation-only change — no code or scripts modified.

New sections added:

• withinDistance (ST_DWithin) — compound input operator with point + distance fields, with both GraphQL and SDK examples
• orderingEquals operator — was implemented but missing from docs
• Type-specific fields table — documents available subfields per geometry type (Point, LineString, Polygon, Multi*, Collection)
• Measurement fields — area, length, perimeter (geodesic, computed client-side)
• Transformation fields — centroid, bbox, numPoints
• Aggregate functions — ST_Extent, ST_Union, ST_Collect, ST_ConvexHull
• Cross-references — links to pgpm Docker skill and codegen ORM patterns (no duplicated content)

Review & Testing Checklist for Human

• Verify withinDistance input shape matches the actual WithinDistanceInput GraphQL type — doc shows { point: GeoJSON, distance: Float }. Check against within-distance-operator.ts in the constructive repo.
• Verify measurement field availability — doc claims area/perimeter on Polygon, length on LineString. Confirm against measurement-fields.ts.
• Check cross-reference relative paths — ../../pgpm/SKILL.md, ../../pgpm/references/docker.md, and ./codegen-orm-patterns.md should resolve correctly from skills/constructive-graphql/references/.

Notes

• The aggregate functions section is intentionally brief since aggregates depend on the Graphile aggregates system being enabled, which is a separate configuration concern.
• Geography Point fields use longitude/latitude instead of x/y — this is documented but worth double-checking against the inflection plugin.

Link to Devin session: https://app.devin.ai/sessions/d0a20ee2e5194f4e8c342c6bc12fea07
Requested by: @pyramation

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