core/tools: show images instead of describing them

New `view_image` reads a local picture (JPEG, PNG, GIF, WebP, sniffed from its
bytes) and hands it to the model as an image part, with an optional `detail`
level — `low` measured 3.4x cheaper on a 1200x900 picture. `read_file` now
points at it instead of returning decode noise.

`fetch_local` returns an image response as the picture itself, and MCP image
content blocks become image parts instead of `[image: ...]` placeholders, so a
screenshot server finally reaches the model.

tests/test_tools/test_fetch_local.py

@@ -2,12 +2,14 @@
 
 from __future__ import annotations
 
+import base64
 import threading
 from http.server import BaseHTTPRequestHandler, HTTPServer
 from typing import TYPE_CHECKING, override
 
 import pytest
 
+from plyngent.agent import ToolOutput
 from plyngent.tools.context import read_only_context
 from plyngent.tools.danger import classify_danger
 from plyngent.tools.net import fetch_local, grant_private_host
@@ -29,6 +31,12 @@ if TYPE_CHECKING:
     from pathlib import Path
 
 
+# A real 1x1 PNG: the body must be sniffed by its bytes, not by the header.
+_PNG_BYTES = base64.b64decode(
+    "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8BQDwAEhQGAhKmMIQAAAABJRU5ErkJggg=="
+)
+
+
 class _Handler(BaseHTTPRequestHandler):
     """Minimal echo server for fetch_local tests."""
 
@@ -66,6 +74,9 @@ class _Handler(BaseHTTPRequestHandler):
         if self.path.startswith("/bin"):
             self._send(200, b"\x00\x01\x02\xffbinary", content_type="application/octet-stream")
             return
+        if self.path.startswith("/png"):
+            self._send(200, _PNG_BYTES, content_type="image/png")
+            return
         if self.path.startswith("/ua"):
             ua = (self.headers.get("User-Agent") or "").encode()
             self._send(200, ua)
@@ -200,6 +211,20 @@ async def test_fetch_get_post_put_delete(workspace: Path, http_server: str) -> N
     assert "status=204" in out_del
 
 
+async def test_an_image_response_comes_back_as_the_picture(workspace: Path, http_server: str) -> None:
+    """A PNG body is handed to the model as an image part, not a hex preview."""
+    del workspace
+    grant_private_host("127.0.0.1", int(http_server.rsplit(":", 1)[1]))
+
+    output: object = await fetch_local.handler(f"{http_server}/png")
+
+    assert isinstance(output, ToolOutput)
+    assert "body_kind=image" in output.text
+    assert "status=200" in output.text
+    part = output.images[0]
+    assert part.image_url.url.startswith("data:image/png;base64,")
+
+
 async def test_fetch_read_only_context_blocks_non_get(workspace: Path, http_server: str) -> None:
     """In a read-only context fetch_local only allows GET; outside it POST works."""
     del workspace