v0.8.6: general mode, clipboard text, PDF text extraction

Working mode: --mode coding|general (CST_MODE, settings.json, /mode,
WebUI run settings). general swaps the default framing for a general
assistant one -- workflow, safety notes, tool wording (run_shell, the
subagents) -- and /status shows it; minimal (the paper profile) stays
byte-frozen. The prompt state carries the mode so a switch leaves a
system note in history.

Clipboard: clipboard_read / clipboard_write tools (16 MiB cap read,
default 64 KiB window; writing is a WRITE permission with a preview) on
pbcopy / wl-copy / xclip / pbpaste / wl-paste / xclip and the Win32
clipboard. In the TUI, /attach paste falls back to clipboard text when
there is no image on it.

PDF: a dependency-free text extractor (src/pdf_text.c): inflate,
Flate/ASCIIHex/ASCII85, PNG/TIFF predictors, layout-independent object
scan, object streams, page tree walk with inherited resources, content
lexer (Tj/TJ/'/" and the Td/TD/T*/Tm spacing heuristics), per-font
ToUnicode CMaps and inline images skipped. Encrypted files and pages
with no text layer are refused with the reason.

Attachments: /attach (a path or files dropped on the terminal) now take
images, PDFs and text. A PDF is stored as its text layer with the mime
still saying application/pdf and its page count; anything else has to be
UTF-8 text. Documents travel inline as text parts, so CST_NO_IMAGES and
text-only models no longer drop them, while images keep their old path.

Tests: test_pdf_text (plain, bare and zlib-framed Flate, inline image,
CID + ToUnicode, object stream, error paths), documents in test_attach,
inlined documents in test_llm_body, mode in test_config.

tests/test_config.c

@@ -106,11 +106,13 @@ static void test_settings(void) {
   snprintf(cfg.provider, sizeof(cfg.provider), "openai");
   snprintf(cfg.model, sizeof(cfg.model), "gpt-5");
   snprintf(cfg.effort, sizeof(cfg.effort), "max");
+  snprintf(cfg.mode, sizeof(cfg.mode), "general");
   CHECK(cst_settings_remember(&cfg) == 0);
 
   CHECK(cst_settings_load(&s) == 0);
   CHECK(strcmp(s.provider, "openai") == 0);
   CHECK(strcmp(s.effort, "max") == 0);
+  CHECK(strcmp(s.mode, "general") == 0);
   CHECK(strcmp(cst_settings_model(&s, "deepseek"), "deepseek-v4-pro") == 0);
   CHECK(strcmp(cst_settings_model(&s, "openai"), "gpt-5") == 0);
   CHECK(s.nmodels == 2);
@@ -145,6 +147,24 @@ static void test_effort(void) {
   free(body);
 }
 
+static void test_mode(void) {
+  CHECK(cst_mode_valid("coding") && cst_mode_valid("general"));
+  CHECK(!cst_mode_valid("code") && !cst_mode_valid(NULL));
+  CHECK(!cst_mode_valid(""));
+  CHECK(strcmp(cst_mode_name("general"), "general") == 0);
+  CHECK(strcmp(cst_mode_name(NULL), CST_DEFAULT_MODE) == 0);
+  CHECK(strcmp(cst_mode_name("nonsense"), "coding") == 0);
+
+  CHECK(cst_settings_remember_mode("general") == 0);
+  cst_settings_t s;
+  CHECK(cst_settings_load(&s) == 0);
+  CHECK(strcmp(s.mode, "general") == 0);
+  /* an unknown value is recorded as the default, not as itself */
+  CHECK(cst_settings_remember_mode("junk") == 0);
+  CHECK(cst_settings_load(&s) == 0);
+  CHECK(strcmp(s.mode, "coding") == 0);
+}
+
 int main(void) {
   CHECK(test_tmpdir(g_dir, sizeof(g_dir), "cst-config") == 0);
   setenv("XDG_CONFIG_HOME", g_dir, 1);
@@ -158,6 +178,7 @@ int main(void) {
   test_creds();
   test_settings();
   test_effort();
+  test_mode();
 
   CHECK(test_rmrf(g_dir) == 0);
   printf("test_config: OK\n");