[spec] Add get-store-categories tool

[baldasseva]

Problem

With the issue #1366, we have introduced categories to MCP search. However there is a 💩 ton of categories and they could change in time, so it is impossible to fit them in a tool description.

Proposed solution

Add a get-store-categories tool that renders the taxonomy from @apify/consts — static, no API call, no cache. Takes an optional group (default USE_CASE), returns the use-case tree (or a flat list for other groups) with id, name and short description, hidden categories excluded. Default-on, unauth-enabled, registered before search-actors in the actors category.

Plan

• Add STORE_CATEGORIES_GET: 'get-store-categories' to HELPER_TOOLS, place it in the actors registry category ahead of search-actors (discover → search), and add it to the unauth-enabled list since it never touches the API.
• Build the taxonomy helpers on top of @apify/consts. A getter that returns the categories of one group, mapped to a plain shape; a tree builder that nests use-case categories by parentCategoryId; and a formatter that renders the tree as an indented Markdown list (- ID — Name: description).
• Implement the tool. Single group argument as a Zod enum over the taxonomy's group values, defaulting to USE_CASE, with the option list generated from the enum (same pattern as search-apify-docs' docSource). Description explains what categories are, that use-case ids are hierarchical and the most specific fitting id should be picked, and — gated on hasTool(search-actors) — that one id goes into search-actors' category. Result is the rendered tree plus structuredContent (group, categories, count, instructions) validated by a new output schema; the search-actors hint in the result text is gated on loadedToolNames. Read-only/idempotent annotations; it cannot fail, so always respondOk.
• Wire the new tool to search-actors that has been set to accept categories in issue [spec] Add use case categories to search-actors tool #1366 .
• Update every place that pins the default tool set. Mode-contract and category-ordering tests, server_card expected names, the unauth test, server-instructions gating test, registration integration cases (expected actors tools, total default count, sibling not.toContain checks), e2e tools-list length assertions, README tool table and unauth allowlist, src/tools/AGENTS.md and the root AGENTS.md tool enumeration (pnpm run check:agents). Decide with the ai-team whether the Claude connector allowlist includes the new tool.
• Add tests, docs, evals, hosted repo. Flag in the PR that tools/list, server_card and the default tool count change, so apify-mcp-server-internal's contract suite needs a matching update.

Alternatives considered

No response