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/agent_store.c

@@ -288,7 +288,10 @@ int cst_agent_store_info(const char *workspace, const char *id,
     cJSON *o = cJSON_Parse(line);
     const cJSON *r = cJSON_GetObjectItemCaseSensitive(o, "role");
     const cJSON *c = cJSON_GetObjectItemCaseSensitive(o, "content");
-    if (cJSON_IsString(r) && strcmp(r->valuestring, "user") == 0 &&
+    const cJSON *k = cJSON_GetObjectItemCaseSensitive(
+        cJSON_GetObjectItemCaseSensitive(o, "meta"), "kind");
+    int note = cJSON_IsString(k) && strcmp(k->valuestring, "note") == 0;
+    if (cJSON_IsString(r) && strcmp(r->valuestring, "user") == 0 && !note &&
         cJSON_IsString(c) && c->valuestring[0]) {
       clip_line(info->first_user, sizeof(info->first_user), c->valuestring);
       have_first = 1;