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

tests/test_mcp.c

@@ -5,6 +5,8 @@
 #define _POSIX_C_SOURCE 200809L
 
 #include "cJSON.h"
+#include "cst_agent.h"
+#include "cst_attach.h"
 #include "cst_mcp.h"
 #include "cst_tools.h"
 #include "os_compat.h"
@@ -18,6 +20,7 @@
 
 static int nfail;
 static char g_tmp[OS_PATH_MAX];
+static cst_agent_t *g_agent;
 
 #define CHECK(cond, msg)                                                       \
   do {                                                                         \
@@ -99,6 +102,7 @@ static int server(void) {
         cJSON_AddItemToArray(tools, tool("mixed", "Mixed content."));
         cJSON_AddItemToArray(tools, tool("hang", "Never answers."));
         cJSON_AddItemToArray(tools, tool("crash", "Exits."));
+        cJSON_AddItemToArray(tools, tool("pic", "Returns images."));
       }
       reply(id, r);
     } else if (strcmp(mt, "tools/call") == 0) {
@@ -136,6 +140,21 @@ static int server(void) {
         cJSON_AddStringToObject(rr, "text", "resource text");
         cJSON_AddItemToArray(c, res);
         reply(id, r);
+      } else if (strcmp(name, "pic") == 0) {
+        cJSON *r = text_result("a chart", 0);
+        cJSON *c = cJSON_GetObjectItemCaseSensitive(r, "content");
+        const char *datas[] = {
+            "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNkYPhfDwAC"
+            "hwGA60e6kgAAAABJRU5ErkJggg==",
+            "AAAA"};
+        for (int i = 0; i < 2; i++) {
+          cJSON *img = cJSON_CreateObject();
+          cJSON_AddStringToObject(img, "type", "image");
+          cJSON_AddStringToObject(img, "mimeType", "image/png");
+          cJSON_AddStringToObject(img, "data", datas[i]);
+          cJSON_AddItemToArray(c, img);
+        }
+        reply(id, r);
       } else if (strcmp(name, "crash") == 0) {
         fprintf(stderr, "fatal: fake crash\n");
         exit(3);
@@ -163,6 +182,7 @@ static int call(const char *tool, const char *k, const char *v,
   cst_tool_ctx_t ctx;
   memset(&ctx, 0, sizeof(ctx));
   ctx.workspace_root = g_tmp;
+  ctx.agent = g_agent;
   memset(r, 0, sizeof(*r));
   return cst_tool_run(&ctx, &c, r);
 }
@@ -217,8 +237,8 @@ int main(int argc, char **argv) {
   cst_mcp_info_t mi[CST_MCP_MAX];
   int n = cst_mcp_info(mi, CST_MCP_MAX);
   const cst_mcp_info_t *f = info_of("fake", mi, n);
-  CHECK(f && f->state == CST_MCP_READY && f->ntools == 6,
-        "fake ready with 6 tools over two pages");
+  CHECK(f && f->state == CST_MCP_READY && f->ntools == 7,
+        "fake ready with 7 tools over two pages");
   const cst_mcp_info_t *rm = info_of("remote", mi, n);
   CHECK(rm && rm->state == CST_MCP_FAILED && strstr(rm->error, "stdio"),
         "url servers unsupported");
@@ -255,8 +275,40 @@ int main(int argc, char **argv) {
   CHECK(r.exit_code == 0 && r.out && strstr(r.out, "first\n[image image/png") &&
             strstr(r.out, "resource text"),
         "content items mapped to text");
+  CHECK(r.nattach == 0, "no session: images stay text");
   cst_tool_result_free(&r);
 
+  cst_agent_config_t acfg;
+  memset(&acfg, 0, sizeof(acfg));
+  snprintf(acfg.workspace_root, sizeof(acfg.workspace_root), "%s", g_tmp);
+  char sdir[1024];
+  snprintf(sdir, sizeof(sdir), "%.900s/cst-mcp-test-%d", g_tmp, (int)getpid());
+  os_mkdir_p(sdir, 0755);
+  snprintf(acfg.session_path, sizeof(acfg.session_path), "%s/s.jsonl", sdir);
+  g_agent = cst_agent_create(&acfg);
+  CHECK(g_agent != NULL, "agent for the session store");
+  call("mcp__fake__pic", NULL, NULL, &r);
+  CHECK(r.exit_code == 0 && r.nattach == 1 && r.attach &&
+            strcmp(r.attach[0].mime, "image/png") == 0 &&
+            strcmp(r.attach[0].origin, "mcp") == 0 &&
+            r.attach[0].width == 1 && r.attach[0].height == 1,
+        "image content stored and attached");
+  CHECK(r.out && strstr(r.out, "a chart\n[image ") &&
+            strstr(r.out, "shown to you as an image") &&
+            strstr(r.out, "not shown:"),
+        "one line per image, bad data falls back to text");
+  if (r.nattach == 1) {
+    char full[OS_PATH_MAX];
+    CHECK(cst_attach_full_path(acfg.session_path, &r.attach[0], full,
+                               sizeof(full)) == 0 &&
+              access(full, R_OK) == 0,
+          "stored file exists");
+  }
+  cst_tool_result_free(&r);
+  cst_agent_destroy(g_agent);
+  g_agent = NULL;
+  test_rmrf(sdir);
+
   volatile int cancel = 1;
   os_set_cancel_flag(&cancel);
   double t0 = os_now_ms();