feat(r): opt-in "maps" capability — plumbing, prompts, docs

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Summary

R side of the opt-in "maps" capability (sibling to #323 for Python; spec in local docs/superpowers/specs/2026-10-12-maps-capability-design.md).

Included here:

• Opt-in plumbing: tools = c("default", "maps"). "default" expands to the default tool set (also fixes the existing "must restate every tool to add one" problem); "maps" is a capability modifier that aborts with a clear error if "visualize" is absent, and is dropped alongside "visualize" when the ggsql package is missing.
• Prompt: the ggsql-spatial.md partial (byte-identical to the Python copy) renders only when maps is enabled. It teaches the ggsql:world choropleth-join pattern, lon/lat point maps, named projections, and guardrails.
• Docs/NEWS: "Maps" section in the visualize vignette + NEWS entry.

Not included yet (needs the in-progress R caching-reader work, which is currently uncommitted in a local checkout of ggsql-caching-reader-r): the spatial-extension pre-warm in viz_caching_reader() and spatial error translation in the viz tool. Until those land, enabling maps gives the LLM spatial guidance but queries fail with ggsql's raw extension error — so do not merge before that follow-up. One open dependency for the follow-up: the extension install location must respect CRAN policy (a user cache dir rather than ~/.duckdb), which may need a small ggsql-r hook.

Verification

• TDD coverage: normalize_tools sentinel expansion and maps-requires-visualize error, check_viz_deps dropping maps, prompt rendering with/without maps.
• devtools::test(filter = "QueryChat") and filter = "QueryChatSystemPrompt": 0 failures.