feat(skillset-update): 新增 skillset-update 技能——一份變更需求批次套用到整個技能組

What（改了什麼）：
- 新增 skills/skillset-update/SKILL.md：五步流程——決策樹確認變更範圍 → 依 Gitea 正本 marketplace 同步全部 domain repo → 每個受影響 domain 各派一個 sub agent 套用變更（含 README「Skills 目錄」同步與三份 manifest 版本遞增）→ 逐技能過 guidelines 審核檢查清單 → 每個受影響 repo 各開一個 PR。
- README.md「Skills 目錄」新增 skillset-update 條目，並同步更新 skill-update 與 skill-delete 的流程描述（marketplace 先行、補 clone、深度刪除驗證）。

Why（為什麼改）：
- 既有 skill-update 一次只能改一個技能；跨多技能、多 domain 的規則變更（如共用流程步驟調整）需要逐 repo 重複操作，容易漏改。
- README 需與技能實際流程保持同步，此為 guidelines 審核檢查清單的必要項目。

How（怎麼改）：
- 依 guidelines.md 準則撰寫：英文 description 含觸發與不觸發時機（單一技能改用 skill-update）、Gitea 操作一律走 jsc-gitea/tools/gitea.sh、提問一律走 jsc-ask:ask、需隔離的步驟標明 MUST run as a sub agent、每步標明完成條件。
- README 條目與各技能一節的敘述對齊 SKILL.md 的實際流程。
- 註：README.md 的三處變更位於同一個 diff hunk、無法乾淨拆分，故整檔併入本群組（本群組為最後一個觸及 README 的 feature 群組）。

Who（影響哪個功能／使用者）：
- 需要對整個 jsc 技能組做橫向規則變更的維護者；單一技能更新的使用者不受影響（仍走 skill-update）。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skills/skillset-update/SKILL.md

@@ -0,0 +1,16 @@
+---
+name: skillset-update
+description: Apply one change request across the whole jsc skill set — multiple skills in multiple domains in one pass. Ask the change details via decision tree, sync every domain repo from the Gitea canonical marketplace, apply the change per affected domain via sub agents, re-check against the guidelines checklist until it passes, then open a PR per affected repo via jsc-git pr. Use when a change spans multiple skills or domains; not for a single skill (use skill-update).
+---
+
+# skillset-update — apply one change across the skill set
+
+Single source of guidelines: [`../../references/guidelines.md`](../../references/guidelines.md).
+
+## Flow
+
+1. Ask for the change details via the `jsc-ask:ask` decision tree: what rule or behavior changes, which skills and which domains are affected. Every option states its impact scope (example: changing a shared flow step touches every skill that calls it). Completion condition: the affected-skill list is agreed with the user.
+2. Query the Gitea canonical marketplace for the authoritative domain list: run `jsc-gitea/tools/gitea.sh api GET /repos/plugins/jsc/raw/.claude-plugin/marketplace.json`. Clone any domain repo missing from the working directory (`gitea.sh clone-url plugins/{domain}`) and pull the rest. Completion condition: every domain repo exists locally and is current.
+3. Apply the change to every affected skill — this step MUST run as a sub agent, one sub agent per affected domain repo: modify SKILL.md and related files and tools, sync that domain README's 「Skills 目錄」 section, and bump the version in all three manifests. Completion condition: every affected domain repo carries the change, the README sync, and the manifest bump.
+4. Check every item of the guidelines.md audit checklist for each touched skill. On any failure, **return to step 1**: ask again and fix, until all items pass.
+5. Call `jsc-git:pr` once per affected domain repo to open a Push Request. Completion condition: every affected repo has a PR URL.