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/tools/fs_tools.c

@@ -14,6 +14,7 @@
 
 #include <regex.h>
 
+#include "cst_attach.h"
 #include "cst_tools.h"
 #include "cst_util.h"
 #include "os_compat.h"
@@ -241,6 +242,47 @@ int cst_tool_read_file2(const cst_tool_ctx_t *ctx, const ctl_tool_call_t *c,
   return tool_take_buf(r, &b, 0);
 }
 
+/* --- view_image ---------------------------------------------------------- */
+
+/* Show an image to the model: the bytes are copied into the session's
+ * attachment store and travel as a multimodal content part of the result. */
+int cst_tool_view_image(const cst_tool_ctx_t *ctx, const ctl_tool_call_t *c,
+                        cst_tool_result_t *r) {
+  const char *path = tool_arg(c, "path");
+  if (!path || !path[0])
+    return tool_set_result(r, "error: missing path", 1);
+  const char *session = ctx->agent ? cst_agent_session_path(ctx->agent) : NULL;
+  if (!session || !session[0])
+    return tool_set_result(r, "error: no session to store the image in", 1);
+  char full[OS_PATH_MAX];
+  if (resolve(ctx, path, full, sizeof(full)) != 0)
+    return tool_set_resultf(r, 1,
+                            "error: path outside workspace or missing parent: "
+                            "%.200s",
+                            path);
+  if (is_dir(full))
+    return tool_set_resultf(r, 1, "error: %.200s is a directory", path);
+
+  char err[256];
+  cst_attachment_t *a = cst_attach_import_file(session, full, err, sizeof(err));
+  if (!a)
+    return tool_set_resultf(r, 1, "error: cannot show %.200s: %s", path, err);
+  snprintf(a->origin, sizeof(a->origin), "view_image");
+  const char *d = tool_arg(c, "detail");
+  if (d && (strcmp(d, "low") == 0 || strcmp(d, "high") == 0))
+    snprintf(a->detail, sizeof(a->detail), "%s", d);
+
+  char desc[128];
+  cst_attach_describe(a, desc, sizeof(desc));
+  cst_buf_t out = {0};
+  cst_buf_printf(&out, "view_image %.200s: %s\n(shown to you as an image)\n",
+                 path, desc);
+  int rc = tool_take_buf(r, &out, 0);
+  r->attach = a; /* one entry; freed as a block by cst_tool_result_free */
+  r->nattach = 1;
+  return rc;
+}
+
 /* --- edit_file ----------------------------------------------------------- */
 
 static size_t count_occ(const char *hay, size_t hn, const char *needle,