Extensions: skills, plugins, MCP, sub-agents; configurable keys

- Skills (SKILL.md discovery, skill tool, /skills /skill)
- plugin.json exec tools with permission prompts, timeouts and workspace trust (/plugins)
- MCP stdio client: JSON-RPC initialize/tools/list/tools/call, mcp__server__tool, /mcp and sidebar status
- task tool with general/explore/plan and .agent/agents/*.md roles, parallel read-only runs, cancel propagation, /agents
- Keymap with Ctrl+X leader, keybinds.json overrides and /keys panel
- Collapsed thinking, throttled streaming Markdown, leaner tool output, parallel read-only tools
- Spawn: close-on-exec pipes and a process-wide fork lock
- Tests for keymap, skills, plugins, MCP, sub-agents, shell compaction; docs/plugins.md

src/agent_store.c

@@ -309,6 +309,21 @@ int cst_agent_store_delete(const char *workspace, const char *id) {
     return -1;
   if (side_path(workspace, id, ".title", p, sizeof(p)) == 0)
     remove(p);
+  /* sub-agent transcripts live in <id>/ */
+  if (side_path(workspace, id, "", p, sizeof(p)) == 0) {
+    DIR *d = opendir(p);
+    if (d) {
+      struct dirent *de;
+      while ((de = readdir(d)) != NULL) {
+        char f[OS_PATH_MAX];
+        if (strncmp(de->d_name, "sub-", 4) == 0 &&
+            os_path_join(f, sizeof(f), p, de->d_name) == 0)
+          remove(f);
+      }
+      closedir(d);
+      rmdir(p);
+    }
+  }
   char cur[128];
   if (cst_agent_store_get_current(workspace, cur, sizeof(cur)) == 0 &&
       strcmp(cur, id) == 0) {