Add machine-readable JSON output for AI agents

[itsflaid]

Summary

• add --json support to init, analyze, ask, doctor, and config model
• emit exactly one parseable JSON document without ANSI or terminal decoration
• return structured ask results with relevant files, model, and token usage
• return structured doctor diagnostics and runtime errors
• keep init JSON mode non-interactive for agent automation
• document agent usage in generated DEVMAP.md, README, PRD, architecture, command, generated-file, and testing docs
• include the updated README positioning and usage content
• verify JSON behavior from the packed npm artifact

Verification

• pnpm test:cli (62/62)
• pnpm build:cli
• pnpm test:package-e2e
• git diff --check

Closes #17