feat(link): 連結一律寫成 [文字](絕對網址)，寫入前先驗證連得到

取消 [[頁名]] 與 [[顯示文字|頁名]] 兩種同 wiki 寫法，不再分「同存取庫」與
「跨存取庫」兩條規則。那種寫法只在自己那個 wiki 內解析，寫錯不報錯，畫面上
看起來像普通文字或死連結，巡不到也修不了。

連結寫進頁面前先過 jsc-gitea 的 link-check.sh，結束碼 0 才寫。驗證一律走 API，
不看網頁狀態碼：私有存取庫的網頁網址對未登入請求一律回 404，拿狀態碼判會把
好連結判成壞的。認證失敗回 7，與死連結的 1 分開，免得金鑰一過期就把還在的頁
整批判死。

skills/learn/SKILL.md

@@ -11,7 +11,8 @@ Close the loop on skill runs: record what a run taught you, consult it before th
 
 - Directory page: `LEARN_CONTENTS`. Content page: `LEARN_{HASH}`, one page per repository.
 - The two pages live in different wikis. `LEARN_{HASH}` goes to `jsc-gitea/tools/gitea.sh wiki-repo LEARN`; `LEARN_CONTENTS` goes to `gitea.sh wiki-repo CONTENTS` (`JSC_WIKI_REPO_CONTENTS`, then `JSC_WIKI_REPO`, then exit 3), which never falls back to the LEARN repo. Exit 3 on either hands the question to `jsc-gitea:wiki`, which owns the resolution order and the wording; exit 2 means the type argument was misspelled, so fix it and rerun.
-- Because they sit in different wikis, the directory row links the lesson page by the absolute URL from `gitea.sh wiki-url <LEARN repo> LEARN_{HASH}`. `[[LEARN_{HASH}]]` resolves only inside one wiki and would dead-link from the directory. `wiki-url` exit 4 means the lesson page is not written yet, so write it first; exit 5 means the page carries no `html_url`, so stop and report it and never assemble the URL by hand; exit 7 or 8 means the token or the API failed, so stop and report that status.
+- Every link this skill writes takes the shape `[{text}]({absolute URL})`. The directory row links the lesson page as `[LEARN_{HASH}](<url>)`, with `<url>` from `gitea.sh wiki-url <LEARN repo> LEARN_{HASH}` — never assembled by hand, and never the same-wiki `[[...]]` form, which resolves inside one wiki only and dead-links from the directory without reporting an error. `wiki-url` exit 4 means the lesson page is not written yet, so write it first; exit 5 means the page carries no `html_url`, so stop and report it and never assemble the URL by hand; exit 7 or 8 means the token or the API failed, so stop and report that status.
+- Check every link before it reaches a page: `jsc-gitea/tools/link-check.sh {url}...`. Only exit 0 permits the write.
 - Compute `{HASH}` from `{owner}/{repo}` with `jsc-gitea/tools/hash-id`. It prints the full 40-character uppercase SHA-1 — no truncation and no prefix rewrite, so never shorten it. Exit 1 means no SHA-1 helper on this machine: stop and report that `sha1sum` or `shasum` has to be installed, and never hand-compute the hash. Exit 2 means the input was empty, so fix the `{owner}/{repo}` parse and rerun.
 - All wiki reads and writes go through `jsc-gitea:wiki`, except the `LEARN_CONTENTS` row, which goes through `jsc-gitea/tools/wiki-contents.sh`.
 
@@ -41,6 +42,16 @@ Run after a skill run that produced a reusable lesson.
      | 7 | The token is invalid or lacks permission, so the old rows are unknown. Stop and report the token problem, and create no page |
      | 8 | Some other API failure. Stop and report that status, and create no page |
 
+   - Check the row's link before writing it. Feed the `wiki-url` output to `jsc-gitea/tools/link-check.sh`; it prints one `{OK|DEAD|SKIP}<TAB>{url}<TAB>{note}` line per URL and resolves Gitea URLs through the API, because a private repo answers a logged-out web request with 404 and would fail a page that is there.
+
+     | Exit | Do |
+     | --- | --- |
+     | 0 | Every link answered. Write the row |
+     | 1 | At least one link is DEAD. Write nothing and report the DEAD lines to the caller |
+     | 2 | No URL reached the script. Pass the URLs and rerun |
+     | 3 | The list holds a Gitea URL but `GITEA_HOST` is unset. Set it and rerun; never skip the check |
+     | 7 | The Gitea token was rejected (HTTP 401/403). Stop and report the token problem. A rejected token makes live pages look missing, and one batch judged on that answer wipes out links that still work |
+
    - Update `LEARN_CONTENTS` in the same pass, and let `jsc-gitea/tools/wiki-contents.sh` do the row work — never hand-edit the directory page. Build one file holding the single row from `templates/learn-contents.md` (the repository name, the absolute link from `gitea.sh wiki-url <LEARN repo> LEARN_{HASH}`, and the update time), then run:
 
      `jsc-gitea/tools/wiki-contents.sh upsert LEARN 1 "{owner}/{repo}" {row file} templates/learn-contents.md`