feat: `cmd skills update` -- record install source so skills can be updated

Problem

cmd skills add does not record where a skill came from. Once installed, there is no manifest mapping skill name to source repo -- nothing in ~/.commandcode/ or the skill directory preserves the owner/repo[/path][@ref] spec used at install time.

Consequence: there is no way to update an installed skill from within the CLI. cmd skills only has add / remove / list (unchanged since v0.18.2). The only update path is:

1. Rediscover the source repo yourself (out of band -- the CLI can't tell you)
2. Re-run cmd skills add <owner/repo/path> -f

For skills installed from multi-skill repos or subpaths (e.g. cmd skills add acme/repo/path/to/skill), rediscovering the exact spec is genuinely error-prone.

Prior art

• Mods already solve this in Command Code itself: cmd mods update reinstalls and reconciles every configured mod package, because mod sources are tracked in settings.json. Skills are the odd one out.
• GitHub CLI shipped gh skill (v2.90.0, April 2026), which records provenance metadata at install time and supports gh skill update [name|--all] across agent hosts.

Proposal

1. At cmd skills add time, write the resolved source spec (owner/repo[/path][@ref] plus installed commit SHA) to a manifest -- either a skills.json alongside the skills directory or a per-skill metadata file.
2. Add cmd skills update [skill-name] and cmd skills update --all: re-fetch from the recorded source and overwrite in place (same semantics as add -f).
3. cmd skills list could then also show source + whether upstream has moved.

Part 1 is the load-bearing piece -- without provenance, no update verb is possible. Backfill for already-installed skills can be manual (prompt on first update, or accept a spec argument to re-pin).

Tested on v1.5.0 (macOS); docs at commandcode.ai/docs/skills confirm only add/list/remove exist.

从现有的 cmd skills add、list 和 remove 入口开始，然后比较 cmd mods update 使用的 settings.json 中的来源处理方式。完成的标准是安装能够保留源规范和 commit SHA，同时 cmd skills update [skill-name] 和 --all 可以重新获取并覆盖已安装的 skills；检查 skills 文档以了解当前的命令界面。