feat: Add Engineer Mode for technical due diligence with 8 new interactive elements

[fogdragon]

Summary

• Engineer Mode: Added a completely new mode targeting professional software engineers doing technical due diligence — a fundamentally different product from the existing Beginner Mode. 8 new interactive element types: ADR Cards, Critical Path Traces, Trade-off Matrix, Scenario Judge, Integration Blueprint, Source File Map, Risk Indicators, Verdict Callout.
• Source File Map: An annotated directory tree classifying 30-50 key files as "framework" (skeleton/architectural) vs "functional" (concrete implementation), with recommended reading order — bridges the gap between "I see the architecture diagram" and "I open the first source file".
• README overhaul: Updated README.md to document both modes with Changelog, and added full Chinese translation (README.zh-CN.md) with cross-language links.

Changes

 SKILL.md                           | 171 ++++++-
 references/content-philosophy.md   |  66 +++
 references/gotchas.md              |  31 ++
 references/interactive-elements.md | 666 ++++++++++++++++++++++++++
 references/main.js                 |  51 ++
 references/styles.css              | 933 +++++++++++++++++++++++++++++++++++++
 README.md                          | rewritten
 README.zh-CN.md                    | new file
 8 files changed, +2134 -23

Test plan

• Use the skill on a codebase with the prompt "Turn this codebase into an engineer mode report" and verify the output contains all 8 interactive elements
• Verify the Source File Map correctly classifies files and provides reading order
• Check that README.md and README.zh-CN.md render correctly with cross-language links
• Confirm Beginner Mode still works as before (no regression)