feat(catalog): add brand, condition, and availability/brand filters

[saraths-stripe]

Add commonly-needed product attributes and search filters to the catalog capability, improving agent discovery and product disambiguation.

Changes

Product schema (types/product.json)

• Add brand (string, optional): manufacturer or brand name recognized by consumers. Enables brand-based filtering and cross-merchant disambiguation.

Variant schema (types/variant.json)

• Add condition (string, optional, open vocabulary): item state. Well-known values: new, refurbished, used.

Search filters (types/search_filters.json)

• Add availability filter (array of strings, OR logic): narrow results to products with at least one variant matching a listed availability status (in_stock, backorder, preorder).
• Add brand filter (array of strings, OR logic, case-insensitive): narrow results by brand name.

Documentation (docs/specification/catalog/)

• Update search.md: document availability and brand filter semantics.
• Update index.md: document brand and condition field conventions.

Design Notes

• All fields are optional and backward-compatible.
• Field names and well-known values use widely adopted commerce conventions (brand, condition, in_stock, preorder, etc.).
• condition and availability.status use open vocabularies (not closed enums), consistent with UCP's schema-authoring conventions for extensibility.
• Filters follow the existing pattern: array values use OR logic within a filter; multiple filters combine with AND logic.
• Businesses MAY ignore filters they do not support and SHOULD notify the platform via a message when doing so.

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