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>

CLAUDE.md

@@ -0,0 +1,11 @@
+# CLAUDE.md
+
+> Claude Code 的仓库级薄入口。进入本仓库后，先读本文，再读 [`AGENTS.md`](AGENTS.md)。
+
+本仓库的权威 agent 规则、文档入口、工作边界和验证方式统一维护在 [`AGENTS.md`](AGENTS.md)。
+
+Claude Code 处理本仓库任务时：
+
+1. 先读取 [`AGENTS.md`](AGENTS.md)。
+2. 再按 `AGENTS.md` 的要求进入 [`docs/00-ai-start-here.md`](docs/00-ai-start-here.md) 和相关模板。
+3. 不在本文重复维护任务流程、编码规则或文档清单，避免和 `AGENTS.md` 漂移。