feat(pr): 目標分支改走階梯，既有 PR 改為校準不重開

What：`pr` 技能新增「PR ladder」一節與階梯形狀的分支命名（`fix` 叫一次 `slugify.sh`，其餘型別叫兩次組出 `{類型}/{功能}/{子功能}`），base 一律由 `base-branch.sh --derive` 推導；新增步驟 6 查目標分支有沒有開啟中的 PR，有就跳到新的步驟 9 比對標題、描述、前置 PR 依賴三項，只有不一樣的那幾項才送 API。`commit` 技能新增步驟 5：認可完成後，目前分支已有 PR 就交給 `pr` 校準。

Why：越級開 PR 與「同一條分支重開第二支 PR」是同一個病灶的兩面——base 與 PR 現況都靠人記。改成先推導、先比對之後，base 由腳本決定，PR 只在真的有差時才更新。

How：呼叫方傳入的 base 仍然照收，但要與推導結果一致；不一致就當成越級擋下，說明正確階梯再問使用者，不會悄悄改目標。校準走 `jsc-gitea` 的 `pr-get` 讀現況、`pr-edit` 一次帶標題與描述、`pr-depend` 補依賴；三項都相同就什麼都不做——每一次多餘的編修都會通知所有審查者，安靜才是對的結果。

Who：`jsc-git:pr` 與 `jsc-git:commit` 兩支技能，以及呼叫它們的 `jsc-sdlc:implement`、`jsc-sdlc:maintain` 與 `jsc-meta` 各技能。

skills/commit/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: commit
-description: Group all pending file changes by conventional type and feature, then commit each group as {type}({scope}): {message}. Before committing, sweep the working tree with jsc-hooks/hooks/comment-scope.sh so no comment carrying document tracking information enters a commit. Message style is full (What/Why/How/Who), brief (one line from git diff), or custom, chosen via decision tree. Use whenever changes must be committed; not for push or PR creation.
+description: Group all pending file changes by conventional type and feature, then commit each group as {type}({scope}): {message}. Before committing, sweep the working tree with jsc-hooks/hooks/comment-scope.sh so no comment carrying document tracking information enters a commit. Message style is full (What/Why/How/Who), brief (one line from git diff), or custom, chosen via decision tree. Once the commits land, an open PR on the current branch gets its title, description, and prerequisite dependency calibrated through jsc-git:pr. Use whenever changes must be committed; not for push or PR creation.
 ---
 
 # commit — group and commit file changes
@@ -15,6 +15,7 @@ description: Group all pending file changes by conventional type and feature, th
    - Done when one of these is true and reported: the sweep exited 0, or every remaining warning is reported with the reason it is a false positive, or the script was not found on this machine.
 3. Group the changes by same type plus same requirement or feature. One group is one commit. Done when each group carries one type and one requirement or feature, and no path sits in two groups.
 4. Commit each group with the format `{type}({requirement or feature}): {message}`. Done when `git status --porcelain` returns empty; report done only then.
+5. Calibrate the open PR of this branch, when there is one: `jsc-gitea/tools/gitea.sh api GET /repos/{owner}/{repo}/pulls?state=open` and match `head.ref` against the current branch. A match → hand the PR number to `jsc-git:pr` step 9, which compares title, description, and prerequisite dependency and updates only what differs. No match → nothing to do here. Done when either the branch has no open PR, or `jsc-git:pr` reported each of the three items as matched or updated.
 
 ## Type table
 
@@ -42,4 +43,4 @@ Ask the user per the `jsc-ask:ask` rules. Skip the question when the question re
 
 1. The grouping and message drafting details **MUST run as a sub agent**. The main agent only confirms the grouping and runs the commits.
 2. Write messages in UTF-8 Traditional Chinese (the type and scope stay in English), per the STE100 output rule.
-3. Never push. Push and PR belong to `jsc-git:pr`.
+3. Never push, and never create a PR. Step 5 only calibrates a PR that already exists; pushing and creating PRs belong to `jsc-git:pr`.