fix(assistant): 根目錄守門補上目錄存在檢查

人在現場那一條路解出根目錄之後，守門只看「非空、結束碼 0、是絕對路徑」三項，攔不住 JSC_HOME 沒設的情況。

實測：JSC_HOME 沒設時，readlink -f "$JSC_HOME/current" 印出 /current、結束碼 0。三項全部符合，直接通關，而 /current 並不存在。之後每一條由它組出來的字面路徑都指向不存在的地方，錯誤要到第一支腳本才浮出來，而且訊息看不出根因。

守門加上第四項：印出來的路徑必須是存在的目錄，用 [ -d ] 在同一個已核准的步驟裡查。檢查只掛在人在現場那一條，排程那一輪照舊一個指令都不跑——它的根目錄是安裝排程的人寫進條目的，錯了就由第一支腳本呼叫失敗來反映。

行為契約的關鍵步驟與完成條件兩列跟著改。

skills/assistant/SKILL.md

@@ -34,7 +34,7 @@ Never work the root out instead. `readlink -f "$JSC_HOME/current"`, `ls -d "$JSC
 
 Take the root once per invocation and reuse that one answer. Never resolve it again per call, never print it as a report line of its own, and never add a tool that prints it. Never test the root with a command either — an unattended round cannot, and the first script call is the test that matters anyway.
 
-An empty token, an empty `readlink` result, or a path that is not absolute means there is no root to work with. Report it, say `jsc-cli:deploy` has to run, take the operation's `aborted` status, and stop. Never fall back to a cache path, and never create the root here. Completion condition: one literal absolute path is in hand and every later command line carries it, or the missing root was reported and the operation stopped.
+An empty token, an empty `readlink` result, a path that is not absolute, or a resolved path that is not an existing directory means there is no root to work with. **That fourth item is the one the other three wave through.** With `JSC_HOME` unset, `readlink -f "$JSC_HOME/current"` prints `/current` and exits 0 — non-empty, absolute, and past every other item — and each literal path built from it then names a place that is not there. So the attended resolve is only accepted once `[ -d "{the path just printed}" ]` says that directory exists, run in the same approved step as the resolve itself. The unattended round tests nothing, exactly as above: its root was written into the entry by whoever installed the schedule, and its first script call is what fails if that root is wrong. Report it, say `jsc-cli:deploy` has to run, take the operation's `aborted` status, and stop. Never fall back to a cache path, and never create the root here. Completion condition: one literal absolute path is in hand and every later command line carries it, or the missing root was reported and the operation stopped.
 
 ## Every script call carries a literal absolute path