docs: improve AGENTS.md and skill trigger coverage

[AmisKwok]

Summary / 摘要

Improve the project's agent guide (AGENTS.md) and CodeBuddy skill trigger coverage, keeping the single-source-of-truth design intact for cross-tool compatibility.

完善项目的 AI 代理指引（AGENTS.md）并扩大 CodeBuddy skill 的自动触发覆盖，同时保持「单一真相源」设计，确保跨工具通用性不受影响。

Changes / 改动内容

AGENTS.md (benefits all AI coding tools: CodeBuddy, Trae, Cursor, Claude Code, Copilot, Codex)

• Architecture: replaced the vague "33 implementation files" note with the real lib/src/ module grouping (interceptors / platform / models / services / utils / ui) and documented the public entry ZeroInspectorKit (init(), wrapApp(), runAppWithInspector()).
• New section — Dependencies and SDK constraints: Dart >=3.11.0 <4.0.0, Flutter >=3.3.0, runtime/dev deps, caret (^) constraint reminder, GPL-3.0 reaffirmation.
• CI: added the previously missing dependabot-pr-bilingual.yml description and the pr-title-check.yml details (allowed types, requireScope: false, ignored labels).
• Release: added local pre-push verification commands (flutter analyze, flutter test, flutter pub publish --dry-run).

.codebuddy/skills/zero-inspector-kit/SKILL.md

• Extended the frontmatter description trigger keywords (Conventional Commits validation, tagging, pana score, flutter analyze/flutter test, docs) to improve auto-load accuracy. No content duplication with AGENTS.md.

Removed empty skill sub-directories

• Deleted references/, scripts/, assets/ under the skill (they were empty and untracked by git). The skill stays a minimal pointer to AGENTS.md, avoiding any split-brain between tool-specific and tool-agnostic guidance.

Why / 设计原则

The skill remains a thin forwarder. All substantive guidance lives in AGENTS.md (read by every major AI tool), so non-CodeBuddy users on the team get the same benefits without a CodeBuddy-specific copy.

skill 仍为极简转发器，所有实质性指引保留在 AGENTS.md（所有主流 AI 工具均读取），确保非 CodeBuddy 组员也能同等受益，避免工具专属副本造成信息分裂。

Checklist / 检查清单

• Commit message follows Conventional Commits (docs:)
• No public API changed (docs only)
• Branch prefix is docs/ (not pushed to main)
• AGENTS.md remains the single source of truth