#2978 stagecraft: read records with data query --single once it ships

Follow-up to swamp-club #2974, which moved stagecraft off swamp data get onto swamp data query with --select content --json. That form prints a results envelope, so subagents and the integration harness read the one result out of it.

When swamp-club #2961 ships data query --single, switch to --single --json, which prints the record as one object and fails on zero or several matches:

• stagecraft/extensions/models/_lib/engine/dispatch.ts, buildSubagentPrompts: the read line and the note on the output shape (and dispatch_test.ts).
• stagecraft/integration/harness.ts, data(): the zero-or-several check becomes swamp's own.
• stagecraft/integration/extension/skill_commands.ts, checkCommand: the data query form.
• Docs: the stagecraft skill (SKILL.md, references/driving.md, references/examples/build-swamp-extension.md), REFERENCE.md and DESIGN.md.

Rebuild the studio bundle (deno task build:studio) after the dispatch.ts change.