move avatar-PNG helpers to hive-core, configurable org avatar

core_avatar_png() and config_org_avatar_png() were hive-sh4re
functions used only by hive-c0re::forge::users. Move them there as
private path-resolution helpers (core_avatar_png_path,
config_org_avatar_png_path), following the existing
std::env::var(...).expect(...) required-env-var style used elsewhere
in hive-c0re.

The org avatar is now independently configurable:
services.hyperhive.c0re.orgAvatarPng (nullable path, default null)
lets an operator override just the agent-configs org avatar PNG
without replacing the whole assets package. Wired via a new
HIVE_ORG_AVATAR_PNG env var that falls back to the bundled PNG when
the option is unset. The core avatar stays under HIVE_ASSETS_DIR,
unchanged.

hive-sh4re/src/assets.rs

@@ -50,23 +50,6 @@ pub fn branding_svg() -> PathBuf {
     dir().join("branding/hyperhive.svg")
 }
 
-/// `$HIVE_ASSETS_DIR/branding/hyperhive.png` — the same mark
-/// rasterised, used by `hive-c0re::forge::push_avatar` to upload
-/// the forge org avatar.
-#[must_use]
-pub fn core_avatar_png() -> PathBuf {
-    dir().join("branding/hyperhive.png")
-}
-
-/// `$HIVE_ASSETS_DIR/branding/agent-configs.png` — secondary org
-/// mark for the `agent-configs/` mirror org. Rendered from
-/// `agent-configs.svg` at asset-build time (was rendered in
-/// `hive-c0re/build.rs`).
-#[must_use]
-pub fn config_org_avatar_png() -> PathBuf {
-    dir().join("branding/agent-configs.png")
-}
-
 /// `$HIVE_ASSETS_DIR/prompts/system.md` — the claude system prompt
 /// template. `hive-agent::prompt::render` filters the role markers
 /// inside it per agent / manager flavor.