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.

app/src/core/ToolProvider.h

@@ -0,0 +1,59 @@
+/* How the front end hands a tool to the engine.
+ *
+ * A tool implemented in C++ is an ordinary cst_tool_def_t: the engine owns a
+ * copy of the definition, so a static (or stack) value is enough, and
+ * `ud`/`run_ex` keep pointing at the front end's state. The callback runs on
+ * the turn's worker thread -- touch Qt objects only through queued signals.
+ *
+ * See app/src/platform/linux/NotifyTool.cpp for a worked example, and
+ * tests/test_embed.c for the same contract without Qt.
+ */
+
+#ifndef CST_APP_TOOLPROVIDER_H
+#define CST_APP_TOOLPROVIDER_H
+
+#include <QString>
+
+#include "cst_tools.h"
+
+/* The engine wrapper of the front end (core/AgentCore.h). Only a pointer
+ * travels through this header, so the class stays incomplete here. */
+class AgentCore;
+
+namespace app {
+
+/* Fill a definition in one place, so every front-end tool carries the same
+ * metadata (class + policy name + platform list + probe). */
+inline void setMeta(cst_tool_def_t *def, const char *name, const char *desc,
+                    cst_tool_perm_t perm, const char *perm_name,
+                    const char *const *platforms,
+                    int (*available)(char *, size_t)) {
+  def->name = name;
+  def->desc = desc;
+  def->perm = perm;
+  def->perm_name = perm_name;
+  def->platforms = platforms;
+  def->available = available;
+}
+
+/* Tools that only work on the phone. */
+extern const char *const kAndroidOnly[];
+/* Tools that only work on a desktop (linux / win / macos). */
+extern const char *const kDesktopOnly[];
+
+/* The platform tools this app provides. Called once the engine exists;
+ * returns how many were registered. */
+int registerPlatformTools(AgentCore *core);
+/* Drop them again (they belong to the owners "app:*"). */
+void unregisterPlatformTools(AgentCore *core);
+
+/* One entry point per platform, implemented by the file under
+ * src/platform/<os>/; ToolProvider.cpp picks one at compile time. */
+int platformRegisterLinux(AgentCore *core);
+int platformRegisterMac(AgentCore *core);
+int platformRegisterWindows(AgentCore *core);
+int platformRegisterAndroid(AgentCore *core);
+
+}  // namespace app
+
+#endif /* CST_APP_TOOLPROVIDER_H */