v0.8.7: tool capability metadata, in-process engine, Qt app, Android engine

The engine can be embedded now instead of only driven from a terminal: the
same objects link into a Qt front end (app/) that owns a cst_agent_t inside
its own process -- no port, no token, no second process -- and that front end
now runs on the desktop and, engine-side, on the phone.

Capability metadata (include/cst_tools.h, src/tools/registry.c):
- cst_tool_def_t gains perm_name, platforms and available (a probe that
  returns a reason), so a permission is addressable by name and a tool can
  say why it cannot run here.
- cst_platform_name(), cst_tool_platform_ok(), cst_tool_available(),
  cst_tool_meta_reload(), cst_tool_perm_name() and CST_PLAT_DESKTOP[].
- cst_tool_register() may take over a built-in name that is not runnable on
  this platform, so a front end replaces it with its own implementation.
- the registry entries carry their class (delete_file -> "delete"), the
  desktop clipboard tools carry a clipboard probe; find/active/run gate on it.
- /api/settings.tools[] exposes perm_name, platforms, available, why and the
  declared parameters.

Engine as a library (Makefile, CMakeLists.txt):
- libcstcore.a from the CORE_OBJ set, and a CMake cstcore target shared by
  the CLI and by app/ (-DCST_BUILD_APP=ON): the engine is compiled once.
- Android branch for the vendored sashidown: a per-ABI archive built with the
  NDK's aarch64-linux-android24-clang wrapper, because CMake's compiler is
  plain clang and keeps --target/--sysroot in its own variables.

Qt front end (app/, new):
- AgentCore wraps one cst_agent_t: turns run on a worker thread, every
  cst_agent_hooks_t callback becomes a Qt signal, and the two hooks that need
  an answer (on_permission, on_ask_user) block the worker until the GUI
  replies; mode, approval and plan mode go through the engine's config.
- ToolProvider hands the front end's tools to the engine (owner "app:*"); the
  Linux one is notification_send with a probe for notify-send/D-Bus.
- Main.qml: transcript, tool blocks, consent dialog (Deny/Once/Always ->
  CST_PERM_*), ask_user, mode switch, Esc to cancel; --selftest stays
  headless and runs the embedding checks without QML.

Android (scripts/, docs/android.md, new):
- scripts/build_third_party_android.sh builds static mbedTLS + libcurl for an
  ABI (CA path /system/etc/security/cacerts); build_android.sh builds
  cst-agent, libcstcore.a and the test binaries for the phone, can push them
  and run the whole suite there.
- sandbox_exec.c keeps the plain exec path on Android: the seccomp supervisor
  compiled in and the probe reported "seccomp", but writes outside the
  workspace were not denied on the device -- claiming a sandbox that does not
  hold is worse than reporting none.
- tests/test_diff.c compares layouts only against real GNU diffutils: toybox's
  diff prints the same hunk header with its own hunk/context rules.

Tests: test_tool_meta (metadata, platform gate, probe reasons, takeover of a
built-in name) and test_embed (in-process embedding contract) are new; the
existing registry/llm_body assertions now follow availability. 38 test
binaries pass on the host, 36/36 on arm64-v8a Android 16, cst-app-selftest
passes in both the standalone app/ build and the repo build.

Version 0.8.7.

tests/test_registry.c

@@ -138,7 +138,13 @@ int main(void) {
   }
 
   cJSON *all = cst_tools_schema_cfg(&cfg);
-  EXPECT(cJSON_GetArraySize(all) == cst_tool_count(), "default: every tool");
+  /* Every tool the host can actually run is offered in the default profile;
+   * one whose platform probe fails (the clipboard without a helper) is not. */
+  int usable = 0;
+  for (int i = 0; i < cst_tool_count(); i++)
+    if (cst_tool_available(cst_tool_at(i), NULL, 0) == 0)
+      usable++;
+  EXPECT(cJSON_GetArraySize(all) == usable, "default: every usable tool");
   EXPECT(schema_has(all, "edit_file") && schema_has(all, "grep") &&
              schema_has(all, "web_fetch"),
          "default schema has new tools");