feat(media): multimodal attachments — image/video inlining per model capabilities

Adds media.rs for per-turn attachment routing, message_builder
partitioning by resolved model capabilities, controller endpoints
for uploads, and server routing for /data/* behind session auth.

See CLAUDE.md §Multimodal attachments for full design.

crates/skald-core/src/session/handler/mod.rs

@@ -37,6 +37,7 @@ mod gate;
 mod interface_tools;
 mod llm_call;
 mod llm_loop;
+pub mod media;
 pub mod message_builder;
 mod messages;
 mod outcome;