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.

include/cst_clipboard.h

@@ -13,6 +13,20 @@ extern "C" {
  * ("no image on the clipboard", "install wl-clipboard", ...). */
 int cst_clipboard_image(char **data, size_t *n, char *err, size_t err_sz);
 
+/* Cap on text moved through the clipboard (read and write). */
+#define CST_CLIPBOARD_TEXT_MAX (1024 * 1024)
+
+/* UTF-8 text on the system clipboard: Wayland (wl-paste), X11 (xclip),
+ * macOS (pbpaste), Windows (CF_UNICODETEXT). Returns 0 with *data malloc'd
+ * and NUL-terminated, or -1 with err filled in ("no text on the clipboard",
+ * "reading the clipboard needs wl-clipboard", ...). */
+int cst_clipboard_text(char **data, size_t *n, char *err, size_t err_sz);
+
+/* Replace the clipboard with UTF-8 text: wl-copy / xclip / pbcopy, or the
+ * Windows clipboard API. Returns 0 on success, else -1 with err. */
+int cst_clipboard_set_text(const char *text, size_t n, char *err,
+                           size_t err_sz);
+
 /* Windows CF_DIB bytes (BITMAPINFOHEADER + pixels) as an image file: a PNG
  * for uncompressed 24/32-bit bitmaps, else a BMP. NULL when malformed. */
 char *cst_dib_to_image(const void *dib, size_t sz, size_t *n);