fix: write issue dependencies to Gitea on push

The local `depends:` graph never reached the tracker. push.py sent dependent
issues in topological order but created no native links, so `native_deps` in
_gitea.py was a reader with no writer and the slugs in `## Depends on` stayed
dead prose for anyone reading the issue in Gitea.

Once an issue has its number, every `depends:` entry that also has one now
becomes a real link: POST /repos/{owner}/{repo}/issues/{index}/dependencies
with the blocker's IssueMeta. Topological order means the blocker is already
numbered, so no second pass is needed. Existing links are read back first, so
a repeat push is a no-op and never 409s; a link that fails anyway warns rather
than aborting a run that has already created issues. `--dry-run` prints the
links it would make and touches nothing.

The `## Depends on` prose is still passed through verbatim — the edge the
tracker acts on is the native link, not the text, which is exactly why the
text can be left alone. Removing a link that disappeared from `depends:` is
out of scope and now says so in push.py's docstring.

Establishes tests/: stdlib unittest, the transport stubbed at _gitea.api, no
network. Run with `python3 -m unittest discover -s tests`.

The POST body shape was confirmed against the instance's own swagger.v1.json
(Gitea 1.26.1), not assumed from upstream docs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

skills/sync/scripts/_gitea.py

@@ -226,6 +226,46 @@ def native_deps(login, base, number):
     return [i["number"] for i in got] if isinstance(got, list) else []
 
 
+def native_dep_pairs(login, base, number):
+    """The same links as {(owner/repo, number)} — what a repeat push compares
+    against so it does not POST a link the tracker already has.
+
+    A bare number is ambiguous the moment a dependency lives in another repo,
+    and IssueMeta lets it, so the repo travels with it. The pair is a transport
+    fact; formatting it as `owner/repo#42` is map.py's job, not this module's."""
+    got = api(login, "%s/issues/%d/dependencies" % (base, number), allow_fail=True)
+    out = set()
+    for i in got if isinstance(got, list) else []:
+        repo = (i.get("repository") or {}).get("full_name") or ""
+        if "number" in i:
+            out.add((repo, int(i["number"])))
+    return out
+
+
+def add_dependency(login, base, number, dep_repo, dep_number, out_root=None):
+    """Make issue `number` depend on `dep_repo#dep_number`. True on success.
+
+    Confirmed against the instance's own swagger.v1.json (Gitea 1.26.1):
+
+        POST /repos/{owner}/{repo}/issues/{index}/dependencies
+        body: IssueMeta — {"index": <int>, "owner": "<owner>", "repo": "<name>"}
+        "Make the issue in the url depend on the issue in the form."
+
+    The URL names the blocked issue and the body the blocker, which is the same
+    direction native_deps reads back ("all issues that block this issue"). A
+    link that already exists answers 409, so a failure here is reported and not
+    fatal: one missing cross-link must not abort a push that has already
+    created issues. Callers pre-filter with native_dep_pairs."""
+    owner, _, name = (dep_repo or "").partition("/")
+    if not owner or not name:
+        return False
+    payload = {"index": int(dep_number), "owner": owner, "repo": name}
+    got = api(login, "%s/issues/%d/dependencies" % (base, number), "POST", payload,
+              payload_name="dep-%d-%d" % (number, dep_number),
+              out_root=out_root, allow_fail=True)
+    return got is not None
+
+
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