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_clipboard.h

@@ -0,0 +1,35 @@
+#ifndef CST_CLIPBOARD_H
+#define CST_CLIPBOARD_H
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Image bytes on the system clipboard (PNG, or BMP from a Windows DIB).
+ * Wayland: wl-paste; X11: xclip; macOS: pngpaste or osascript; Windows: the
+ * clipboard API. Returns 0 with *data malloc'd, or -1 with err filled in
+ * ("no image on the clipboard", "install wl-clipboard", ...). */
+int cst_clipboard_image(char **data, size_t *n, char *err, size_t err_sz);
+
+/* Windows CF_DIB bytes (BITMAPINFOHEADER + pixels) as an image file: a PNG
+ * for uncompressed 24/32-bit bitmaps, else a BMP. NULL when malformed. */
+char *cst_dib_to_image(const void *dib, size_t sz, size_t *n);
+
+#define CST_PASTE_MAX_PATHS 8
+#define CST_PASTE_PATH_MAX 1024
+
+/* Split pasted text into file paths the way terminals paste dropped files:
+ * whitespace- or newline-separated, '...' / "..." quoting, file:// URIs
+ * (percent-decoded) and, outside Windows, backslash escapes. Returns the
+ * count, or -1 when the text is empty or holds more than max paths. Whether
+ * the paths exist is up to the caller. */
+int cst_paste_paths(const char *text, size_t len,
+                    char out[][CST_PASTE_PATH_MAX], int max);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif