Add harness coding docs for SoftBox (Go + Gio dual-build)

Initialize the full harness coding document set from the
harness_coding_docs template, customized for the SoftBox project:

- Vision, requirements, tech stack (modern Go 1.25 + Gio v0.10.1;
  Win7 legacy Go 1.20.14 + Gio v0.6.0), architecture, coding rules
- Protocol contracts (signed catalog, package protocol v1, Ed25519
  license, events, CLI) and Gio view structure
- Roadmap Phase 0-6 with 20 suggested tasks; T-001 (monorepo
  skeleton) filed and ready to claim
- Agent entry points (AGENTS.md, docs/00-ai-start-here.md),
  context manifest, governance scripts and tests
- Merge Go gitignore with harness rules; keep go.work tracked

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitattributes

@@ -0,0 +1,12 @@
+# 强制所有文本文件在仓库和检出时统一用 LF，避免 Windows 编辑器/工具把行尾
+# 存成 CRLF，导致每次 diff 满屏“全文件改动”淹没真实改动。
+* text=auto eol=lf
+
+# 二进制文件不做行尾转换（防御性）。
+*.png binary
+*.jpg binary
+*.jpeg binary
+*.gif binary
+*.ico binary
+*.pdf binary
+*.zip binary