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

Makefile

@@ -46,7 +46,7 @@ endif
 PKG_CFLAGS := $(CURL_CFLAGS) $(CURSES_CFLAGS)
 PKG_LIBS := $(CURL_LIBS) $(CURSES_LIBS)
 
-CPPFLAGS += -Iinclude -I$(SD_DIR)/include -I$(CJSON_DIR) -D_XOPEN_SOURCE_EXTENDED $(PKG_CFLAGS)
+CPPFLAGS += -Iinclude -I.gen -I$(SD_DIR)/include -I$(CJSON_DIR) -D_XOPEN_SOURCE_EXTENDED $(PKG_CFLAGS)
 LDFLAGS += -L$(SD_DIR)
 LDLIBS += -lsashidown -lm $(PKG_LIBS) $(PTHREAD_LIBS)
 
@@ -60,7 +60,7 @@ CORE_SRC = \
 	src/sandbox_exec.c \
 	src/keymap.c \
 	src/skills.c \
-	src/extensions.c src/plugins.c src/mcp.c src/subagent.c \
+	src/extensions.c src/plugins.c src/mcp.c src/subagent.c src/attach.c src/clipboard.c \
 	src/session.c \
 	src/tools/tools.c \
 	src/tools/patch.c \
@@ -78,12 +78,13 @@ CORE_SRC = \
 	src/json_baseline.c \
 	src/creds.c \
 	src/mm_tool.c \
-	src/models.c \
+	src/models.c src/model_registry.c \
 	src/metrics.c
 
 SRC = $(CORE_SRC) src/textsel.c src/tui.c src/main.c
 OBJ = $(SRC:.c=.o)
-HDRS = $(wildcard include/*.h) $(wildcard src/tools/*.h) $(CJSON_DIR)/cJSON.h
+HDRS = $(wildcard include/*.h) $(wildcard src/tools/*.h) $(CJSON_DIR)/cJSON.h \
+	.gen/models_builtin.inc
 
 TESTS = \
 	tests/test_ctl_frontier \
@@ -93,9 +94,11 @@ TESTS = \
 	tests/test_replay_real \
 	tests/test_mm_tool \
 	tests/test_llm_body \
+	tests/test_prefix_stable \
 	tests/test_sse_decode \
 	tests/test_apply_patch \
 	tests/test_session_roundtrip \
+	tests/test_attach \
 	tests/test_shell \
 	tests/test_diff \
 	tests/test_tools_fs \
@@ -103,6 +106,7 @@ TESTS = \
 	tests/test_undo \
 	tests/test_config \
 	tests/test_models \
+	tests/test_model_registry \
 	tests/test_sandbox \
 	tests/test_keymap \
 	tests/test_textsel \
@@ -144,6 +148,11 @@ FORCE:
 cst-agent: $(OBJ) $(SD_DIR)/libsashidown.a
 	$(CC) $(CFLAGS) -o $@ $(OBJ) $(LDFLAGS) $(LDLIBS)
 
+# Built-in model data as a C byte list (included by model_registry.c).
+.gen/models_builtin.inc: src/models_builtin.json
+	@mkdir -p .gen
+	od -An -v -tx1 $< | sed 's/\([0-9a-f][0-9a-f]\)/0x\1,/g' > $@.tmp && mv $@.tmp $@
+
 %.o: %.c $(HDRS)
 	$(CC) $(CFLAGS) $(CPPFLAGS) -c -o $@ $<
 
@@ -190,7 +199,7 @@ demo: cst-agent
 	./cst-agent --demo-ctl --workspace .
 
 clean:
-	rm -f $(OBJ) cst-agent $(TESTS)
+	rm -f $(OBJ) cst-agent $(TESTS) .gen/models_builtin.inc
 
 install: cst-agent
 	install -d "$(DESTDIR)$(PREFIX)/bin"