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

CMakeLists.txt

@@ -92,6 +92,8 @@ set(CST_CORE_SOURCES
   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
@@ -110,8 +112,18 @@ set(CST_CORE_SOURCES
   src/creds.c
   src/mm_tool.c
   src/models.c
+  src/model_registry.c
   src/metrics.c
 )
+
+# Built-in model data as a C byte list (included by model_registry.c).
+set(CST_GEN_DIR "${CMAKE_CURRENT_BINARY_DIR}/gen")
+set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS
+  "${CMAKE_CURRENT_SOURCE_DIR}/src/models_builtin.json")
+file(READ "${CMAKE_CURRENT_SOURCE_DIR}/src/models_builtin.json" _cst_models_hex HEX)
+string(REGEX REPLACE "([0-9a-f][0-9a-f])" "0x\\1," _cst_models_hex "${_cst_models_hex}")
+file(WRITE "${CST_GEN_DIR}/models_builtin.inc" "${_cst_models_hex}\n")
+include_directories("${CST_GEN_DIR}")
 if(WIN32)
   # MinGW has no <regex.h>: musl's TRE engine, UTF-8 aware (vendor/tre-regex)
   include_directories(vendor/tre-regex)
@@ -257,9 +269,10 @@ foreach(t ${_CST_SMALL_TESTS})
 endforeach()
 
 # Tests that link the whole core (no TUI).
-foreach(t test_llm_body test_sse_decode test_apply_patch test_session_roundtrip
+foreach(t test_llm_body test_prefix_stable test_sse_decode test_apply_patch
+          test_session_roundtrip test_attach
           test_shell test_diff test_tools_fs test_registry
-          test_undo test_config test_models test_sandbox
+          test_undo test_config test_models test_model_registry test_sandbox
           test_keymap test_shell_compact test_skills
           test_plugins test_mcp test_subagent test_lowil
           test_win_utf8 test_os_dir test_tmpdir)