Add image assets embeddable in artifact content

Agents can now put images inside markdown/HTML artifacts instead of only
posting them as standalone artifacts.

- New Asset model: an image uploaded ahead of the artifact that embeds it.
- POST /api/assets uploads an image (image/* only, 50 MB cap) and returns
  a stable URL the agent references normally, e.g. ![chart](url). No custom
  syntax; existing markdown/fragment rendering handles it.
- GET /assets/{id} serves the image, owner-scoped (404 for other users).
- On artifact creation the content is scanned for asset URLs and matching
  assets are attached to it, so the image dies with the artifact it belongs
  to. Uploads never referenced are swept after a 24h grace period.
- Quota: assets count toward the owner's quota at upload time and can
  trigger eviction; evicting an artifact frees its assets too, and usage
  now sums artifacts plus assets.
- daybook_post.py gains --upload-image, which prints the URL to embed.
- Docs and SKILL.md cover the upload-then-reference workflow.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

skills/daybook-posting/scripts/daybook_post.py

@@ -49,6 +49,19 @@ def main():
         )
         return 2
 
+    if args.upload_image:
+        try:
+            response = upload_asset(base_url, token, Path(args.upload_image))
+        except ValueError as exc:
+            print(f"error: {exc}", file=sys.stderr)
+            return 2
+        except (HTTPError, URLError) as exc:
+            print_http_error(exc)
+            return 1
+        print(json.dumps(response, indent=2))
+        print(response.get("url", ""))
+        return 0
+
     try:
         if args.path:
             resolve_auto_path(args)
@@ -87,11 +100,12 @@ def parse_args():
     parser.add_argument("--source-prompt", default="", help="Prompt/instruction that produced the artifact.")
     parser.add_argument("--source-type", choices=["agent", "person", "system"], default="agent")
     parser.add_argument("--as-markdown", action="store_true", help="Wrap --file content in a Markdown fenced code block instead of uploading it.")
+    parser.add_argument("--upload-image", help="Upload an image for embedding in artifact content. Prints the URL to reference in markdown/HTML.")
     parser.add_argument("--check", action="store_true", help="Fetch /api/capabilities and exit.")
     parser.add_argument("--dry-run", action="store_true", help="Print request details without posting.")
     args = parser.parse_args()
 
-    if args.check:
+    if args.check or args.upload_image:
         return args
 
     if args.path:
@@ -370,6 +384,28 @@ def content_type_for(path):
     return mimetypes.guess_type(path.name)[0] or "application/octet-stream"
 
 
+def upload_asset(base_url, token, path):
+    """Upload an image and return its Daybook URL for embedding in content."""
+    if not path.is_file():
+        raise ValueError(f"image file does not exist: {path}")
+
+    boundary = f"----daybook-{uuid.uuid4().hex}"
+    chunks = [
+        f"--{boundary}\r\n".encode("utf-8"),
+        f'Content-Disposition: form-data; name="file"; filename="{path.name}"\r\n'.encode("utf-8"),
+        f"Content-Type: {content_type_for(path)}\r\n\r\n".encode("utf-8"),
+        path.read_bytes(),
+        b"\r\n",
+        f"--{boundary}--\r\n".encode("utf-8"),
+    ]
+    request = Request(f"{base_url}/api/assets", data=b"".join(chunks), method="POST")
+    request.add_header("Content-Type", f"multipart/form-data; boundary={boundary}")
+    request.add_header("Authorization", f"Bearer {token}")
+    request.add_header("Accept", "application/json")
+    with urlopen(request, timeout=60) as response:
+        return json.loads(response.read().decode("utf-8"))
+
+
 def request_json(method, url, token=""):
     request = Request(url, method=method)
     request.add_header("Accept", "application/json")