Feature: Add structured JSON output mode for all tools

[hoangsonww]

Problem

All tools currently return human-formatted tables/text only. That works for interactive assistants but blocks downstream automation (dashboards, CI gates, historical trend storage) because consumers have to parse display text.

Proposal

Add an optional output_format argument to every tool ("text" | "json", default "text").

When output_format="json", each tool should return a stable typed payload with:

• metadata (tool name, repo root, time window, generated_at)
• summary (top-level metrics)
• items (tool-specific rows/records)
• warnings (partial data, truncation, validation hints)

Keep current text rendering unchanged for default mode.

Scope

• Extend all 8 tool schemas with output_format.
• Introduce shared JSON response contracts in a central module.
• Keep formatter layer for text mode and add JSON serializer helpers.
• Update smoke test and integration tests to validate both modes.
• Add docs/examples showing how to consume JSON mode in scripts.

Acceptance Criteria

• Every tool supports output_format="json" without breaking existing text behavior.
• JSON payloads are deterministic and schema-consistent across tools.
• npm run smoke exercises both text and json modes.
• README/docs include examples and migration notes.
• New tests validate JSON shape and key metric correctness for each tool.