v0.7.0: image input, model registry, prefix-cache savings

Images
- vision gate: text-only models get a note instead of image parts and no
  view_image tool; unknown models retry once without images on HTTP 400
- paste: Ctrl+V and /attach paste read the clipboard (wl-paste, xclip,
  pngpaste/osascript, Win32 PNG or CF_DIB converted to PNG); pasting image
  file paths attaches them
- MCP image content is stored in the session and sent with the tool result
- attach: rel path validation, base64 decode, /retry keeps images

Model registry
- built-in src/models_builtin.json embedded at build time, merged with
  <config>/models.json and cached GET /models metadata (context, vision,
  pricing); most specific rule wins per field
- /model labels, /status source, --list-models [provider]

Cache and cost
- stable request prefix (frozen tools, notes as messages), DeepSeek prefix
  completion, OpenAI/Anthropic cache hints, /cost and hit rate in the status

include/cst_keymap.h

@@ -64,6 +64,7 @@ typedef enum {
   CST_KA_COMPACT,
   CST_KA_COPY,
   CST_KA_EXPORT,
+  CST_KA_PASTE_IMAGE,
   /* model */
   CST_KA_MODEL,
   CST_KA_PROVIDER,