feat(skills): support shared ~/.agents/skills root

[k9ight000]

Summary

• prefer the shared ~/.agents/skills root when it already exists while retaining the legacy Codex++ source as a fallback
• inventory shared Skills independently of Codex++ state and distinguish shared entries from Codex++-managed entries in the UI
• allow enable/disable for existing shared Skills without overwriting or archiving them
• write official [[skills.config]] entries for shared-mode toggles
• archive managed versions before replacement and roll back failed updates
• document the shared-root behavior and data location

Verification

• cargo test -p codex-plus-core --lib (303 passed)
• Skill-focused Rust tests (22 passed)
• manager Rust tests (63 passed)
• Windows integration tests (22 passed)
• frontend tests (118 passed)
• npm run check
• npm run vite:build
• cargo clippy -p codex-plus-core -p codex-plus-manager --all-targets
• git diff --check
• Rust formatting check for modified files

Related to #1190.

[BigPizzaV3]

请补充 Skill 根目录切换时的归属保护：出现 后会切换源目录，但现有 只按 ID 记录，旧私有 Skill 与同名共享 Skill 可能被误判为 managed，进而允许覆盖或归档。需要记录源根、迁移或冲突处理，并补跑 CI。

[BigPizzaV3]

更正：请补充 Skill 根目录切换时的归属保护：出现 ~/.agents/skills 后会切换源目录，但现有 skills.json 只按 ID 记录，旧私有 Skill 与同名共享 Skill 可能被误判为 managed，进而允许覆盖或归档。需要记录源根、迁移或冲突处理，并补跑 CI。

[BigPizzaV3]

请补充共享 ~/.agents/skills 根目录切换时的归属保护：现有 skills.json 只按 ID 记录，同名的旧私有 Skill 与共享 Skill 可能被误判为 managed，进而被覆盖或归档。请记录来源根目录并补同名冲突、切换根目录和启停回归测试，同时触发 CI。

[BigPizzaV3]

当前分支与 main 冲突，且改动同时涉及共享 skills 根目录、UI 状态和归档/回滚。请基于最新 main rebase，确认不会覆盖用户已有 ~/.agents/skills 内容，并补跑三平台 CI 与共享/本地 Skill 升级回滚测试；完成后再复审。

[BigPizzaV3]

关闭理由：分支已与当前 main 冲突（mergeable=CONFLICTING），且自 8-30 我的审查意见（共享 skills 根的归属保护/迁移）后作者未再更新、未 rebase、未新增提交。若计划继续，请基于最新 main rebase 并处理同名 skill 冲突后重开。此关闭随时可重开。