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.

Makefile

@@ -120,6 +120,10 @@ CORE_SRC = \
 
 SRC = $(CORE_SRC) src/textsel.c src/tui.c src/main.c
 OBJ = $(SRC:.c=.o)
+# The core as a static library: what the Qt app embeds in-process (app/), so
+# the GUI links the same engine the CLI/TUI/WebUI run. No TUI, no main.
+CORE_OBJ = $(CORE_SRC:.c=.o)
+CST_CORE_LIB = libcstcore.a
 HDRS = $(wildcard include/*.h) $(wildcard src/tools/*.h) $(CJSON_DIR)/cJSON.h \
 	.gen/models_builtin.inc .gen/web_assets.inc
 
@@ -141,6 +145,8 @@ TESTS = \
 	tests/test_diff \
 	tests/test_tools_fs \
 	tests/test_registry \
+	tests/test_tool_meta \
+	tests/test_embed \
 	tests/test_undo \
 	tests/test_config \
 	tests/test_models \
@@ -171,7 +177,11 @@ TEST_OS_SRC = src/os_compat.c src/os_win.c src/util.c
 
 .PHONY: all clean check demo sashidown-lib FORCE cmake-gcc cmake-clang asan install uninstall
 
-all: sashidown-lib cst-agent
+all: sashidown-lib cst-agent $(CST_CORE_LIB)
+
+$(CST_CORE_LIB): $(CORE_OBJ) FORCE
+	rm -f $@
+	$(AR) rcs $@ $(CORE_OBJ)
 
 cmake-gcc:
 	cmake -B build-gcc -DCMAKE_C_COMPILER=gcc -DCST_BUILD_TUI=ON
@@ -257,7 +267,8 @@ demo: cst-agent
 	./cst-agent --demo-ctl --workspace .
 
 clean:
-	rm -f $(OBJ) cst-agent $(TESTS) .gen/models_builtin.inc .gen/web_assets.inc
+	rm -f $(OBJ) cst-agent $(CST_CORE_LIB) $(TESTS) .gen/models_builtin.inc \
+		.gen/web_assets.inc
 
 install: cst-agent
 	install -d "$(DESTDIR)$(PREFIX)/bin"