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

src/keymap.c

@@ -48,6 +48,8 @@ static const cst_key_action_info_t INFO[CST_KA_COUNT] = {
     [CST_KA_COPY] = {"copy", "Session", "copy the last reply", "/copy"},
     [CST_KA_EXPORT] = {"export", "Session", "export the transcript",
                        "/export"},
+    [CST_KA_PASTE_IMAGE] = {"paste_image", "Session",
+                            "attach the clipboard image", "/attach paste"},
     [CST_KA_MODEL] = {"model", "Model", "pick the model", "/model"},
     [CST_KA_PROVIDER] = {"provider", "Model", "switch provider", "/provider"},
     [CST_KA_EFFORT] = {"effort", "Model", "reasoning effort", "/effort"},
@@ -89,6 +91,7 @@ static const struct {
     {CST_KA_COMPACT, "<leader> c"},
     {CST_KA_COPY, "<leader> y"},
     {CST_KA_EXPORT, "<leader> x"},
+    {CST_KA_PASTE_IMAGE, "ctrl+v"},
     {CST_KA_MODEL, "<leader> m"},
     {CST_KA_PROVIDER, "<leader> p"},
     {CST_KA_EFFORT, "<leader> e"},