lark-cli update currently discovers official skills by parsing unstable human-oriented `skills add --list` output. This prefers the stable official JSON index for skills discovery, while preserving the existing CLI-list fallback and full-install fallback for resilience.
Changes:
- Add official skills index JSON parsing in `internal/skillscheck/sync.go`
- Prefer JSON index discovery before existing CLI list parsing in `internal/skillscheck/sync.go`
- Add reason-chain details when both discovery layers fall back to `fallbackFullInstall`
- Add bounded HTTPS fetch for `https://open.feishu.cn/.well-known/skills/index.json` in `internal/selfupdate/updater.go`
- Add unit tests for parser behavior, discovery fallback order, and fallback detail reasons in `internal/skillscheck/sync_test.go`
Co-authored-by: zhaoyukun.yk <zhaoyukun.yk@bytedance.com>
