fix: add pillow-heif for hermes-agent 0.21.3

Upstream 0.21.3 promotes `pillow-heif>=1.4.0,<2` to a direct dependency
(HEIF/HEIC/AVIF decode for the vision tools), so the wheel metadata now
requires it and `pythonRuntimeDepsCheck` fails with:

    Checking runtime dependencies for hermes_agent-0.21.3-py3-none-any.whl
      - pillow-heif not installed

Changes in package.nix:

- Add `pillow-heif` to the runtime dependencies next to `pillow`.
- Override nixpkgs' `pillow-heif` 1.2.0 up to 1.4.0. 1.4.0 is the newest
  release that still accepts nixpkgs' libheif 1.20.2 (1.5.0 requires
  libheif >= 1.23.1, so it cannot be used with the pinned nixpkgs).

Verification:

- `nix build .#hermes-agent`: builds; previously the same command failed in
  `pythonRuntimeDepsCheck`.
- `nix path-info -r` on the result lists python3.12-pillow-heif-1.4.0.
- pillow-heif 1.4.0 built from source against system libheif passes its own
  test suite: 664 passed, 8 skipped.

package.nix

@@ -50,6 +50,19 @@ let
         };
         patches = [ ];
       });
+      pillow-heif = prev.pillow-heif.overridePythonAttrs (_old: rec {
+        # hermes-agent >=0.21.3 requires pillow-heif>=1.4.0,<2 (HEIF/HEIC/AVIF
+        # decode for the vision tools); nixpkgs has 1.2.0. 1.4.0 is the newest
+        # release that still accepts nixpkgs' libheif (1.20.2) — 1.5.0 needs
+        # libheif >=1.23.1. Upstream test suite passes against system libheif.
+        version = "1.4.0";
+        src = fetchFromGitHub {
+          owner = "bigcat88";
+          repo = "pillow_heif";
+          tag = "v${version}";
+          hash = "sha256-EaislmA4v2qKCDQ87I85Pn8IlS4VJWyNXkITipKSBC8=";
+        };
+      });
       firecrawl-py = prev.firecrawl-py.overridePythonAttrs (_old: rec {
         # hermes-agent >=0.4.0 requires firecrawl-py >=4.16.0; nixpkgs builds from GitHub
         version = "4.16.0";
@@ -252,6 +265,8 @@ pythonPackages.buildPythonApplication {
     pyjwt
     cryptography
     pillow
+    # Vision — HEIF/HEIC/AVIF decode for iPhone photos (upstream >=0.21.3)
+    pillow-heif
     # Messaging
     python-telegram-bot
     discordpy