feat: add --post-url to fetch an arbitrary single post or reel

Lets an out-of-band link (shared by someone, not one of the tracked
profiles) be pulled in directly by URL, filed under its owner's account
like any other post. Bypasses profile planning, archive-db seeding, and
the --min-interval floor entirely, since it's a single request rather
than a recurring surface to budget against.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011qAds5qr7nZRq5R4yAuxUk

scripts/test_gdl_sync.py

@@ -209,6 +209,29 @@ class Publishing(unittest.TestCase):
         self.assertIn("--dry-run", cmd)
 
 
+class PostUrl(unittest.TestCase):
+    """--post-url: a one-off fetch outside the tracked profile list, whose
+    owning account is only known mid-extraction -- same reasoning as
+    highlights, so it must be exempted from the same forced-destination rule."""
+
+    def test_config_keys_a_username_directory(self):
+        config = gdl.build_config("1M", [6.0, 10.0], [3.0, 6.0])
+        for kind in ("post", "reel"):
+            self.assertEqual(
+                config["extractor"]["instagram"][kind]["directory"],
+                ["{username}"])
+
+    def test_destination_is_not_forced_like_posts_and_reels(self):
+        staging = Path("/stage")
+        for subcategory in ("post", "reel"):
+            src = gdl.Source(subcategory, "https://www.instagram.com/p/ABC/",
+                             "", subcategory)
+            cmd = gdl.gdl_command(src, staging, Path("/cfg.json"), "chrome:x",
+                                  None)
+            self.assertIn(str(staging), cmd)
+            self.assertNotIn(str(staging / subcategory), cmd)
+
+
 class UrlsFile(unittest.TestCase):
     def test_reads_every_form_a_person_might_paste(self):
         with tempfile.TemporaryDirectory() as d: