Native image prompts for ACP agents + OMP skills #76

Summary

Expansion beyond legacy: send images as ACP image blocks to agents that accept them, and surface OMP skills in the chat.

Context

• 44madfire/orca#20 Phase C. Legacy OMP never sent native images (src/shared/agent-image-paste.ts:25-27 @76d1b7b); images reach OMP as @path mentions, which OMP commands, options and composer over ACP #71 keeps. OMP advertises image: true (packages/coding-agent/src/modes/acp/acp-agent.ts:649 @53f253f); Orca rejects non-text prompts (src/main/acp/acp-structured-turns.ts:36, src/main/acp/acp-structured-session-adapter.ts:147, capability src/main/acp/acp-structured-agent-definitions.ts:27 @76d1b7b).
• Skills are idle for OMP in legacy chat (src/renderer/src/components/native-chat/use-native-chat-skills.ts:41,102 @76d1b7b); OMP ACP runs skill commands itself.

Scope

• Generic ACP image blocks (bounded size, mime allow-list) when a registration declares imagePrompts; OMP declares it.
• OMP skills via advertised commands in the slash menu.

Non-goals

• No parity gating; no change to @path behavior for agents without image input.

Depends on

• New OMP chats default to ACP when the provider satisfies orca-native-chat-v1 #74

Parity rows owned

• None (expansion rows P-IMAG-01, P-SKIL-01).

Acceptance criteria

• A pasted image reaches OMP as an image block and renders in history.
• Agents without image input keep the @path behavior.
• OMP skills are discoverable and runnable from the chat.

Required tests

• Oversized/unsupported image refused before send; image + text ordering; skills list refresh on available_commands_update.

Base branch

acp-migration (44madfire/orca).