fix(overwrite): 只有頁面真的不存在才建新頁

- What：worklog、learn、report 三支技能共六處讀取分流改寫，只有結束碼 4 才建新頁；
  結束碼 7 與 8 一律中止，一個字都不寫。三份目錄頁樣板補上寫入語意，明寫禁止整頁覆蓋、
  不得改動別人的列。usage-stats.sh 的參數護欄改成明確回傳結束碼 2。
- Why：原本把「讀失敗」與「頁面不存在」當成同一件事。金鑰失效時讀取回 7，技能卻讀成
  「這頁還沒有」，於是照樣板建一份新頁蓋回去。整份工作日誌會被這一筆條目取代，
  既有教訓會被清成空表，報表則產出一份說「這段期間沒有工作」的假數字。
  這些內容只活在 wiki 上，蓋掉就救不回來，所以這是本輪最要緊的一項。
- How：每一處讀取都先看結束碼再決定動作，並在技能文件裡列成表格：
  0 接在既有內容後面附加，4 才從樣板建頁，5 缺網址就中止，7 金鑰或權限問題就中止，
  8 其他 API 失敗就中止。目錄頁一律先讀整頁再改那一列。
  stats 也補上分流，參數錯誤不再被讀成「零次」。
- Who：worklog、learn、report、stats 四支技能的 wiki 讀寫與統計輸出。

skills/stats/SKILL.md

@@ -17,5 +17,11 @@ Data is recorded continuously by `jsc-hooks/hooks/skill-usage.sh` under `$JSC_HO
 
 ## Reporting
 
-1. Run the tool directly and present the output as a table. Done when every line the tool printed appears as one table row.
-2. When the tool prints no rows, explain that `jsc-hooks` must be installed and wired first via `jsc-hooks:hooks-install`. Done when that instruction is reported and no table is shown.
+1. Run the tool directly and branch on its exit code — never read the printed lines without it.
+
+   | Exit | Do |
+   | --- | --- |
+   | 0 | Present every printed line as one table row. No line printed is still exit 0: the data file under `$JSC_HOME/usage/` does not exist yet, which is a count of zero, not a failure — say the counts are zero and that `jsc-hooks` has to be installed and wired via `jsc-hooks:hooks-install` before anything is recorded, and show no table |
+   | 2 | The subcommand or the `--cli` argument was rejected — the subcommand is neither `skills` nor `chains`, or `--cli` came with no value. Fix the argument and rerun. Never rerun the same command unchanged, and never report the counts as zero: nothing was read |
+
+   Done when the exit code was read and the branch it names was taken.