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_agent.h

@@ -47,15 +47,18 @@ int cst_protocol_is_text(cst_protocol_t p);
 /* Eager protocols execute a call during the stream, at confirmation time. */
 int cst_protocol_is_eager(cst_protocol_t p);
 
-/* One image carried by a message (see cst_attach.h). The bytes live beside
- * the session; this is the metadata plus the name of the stored copy. */
+/* One file carried by a message (see cst_attach.h): an image, or a document
+ * whose stored payload is its text. The payload lives beside the session;
+ * this is the metadata plus the name of the stored copy. */
 typedef struct {
   char id[32];      /* unique inside the session ("img-1a2b3c4d") */
   char name[256];   /* display name (original file name) */
-  char mime[32];    /* image/png, image/jpeg, image/gif, image/webp, image/bmp */
+  char mime[32];    /* image/png, ...; application/pdf for a PDF whose text
+                     * layer the stored file holds; text/plain for text */
   char rel[256];    /* file name inside the session's attachment directory */
-  long long bytes;  /* stored size */
+  long long bytes;  /* stored size (the text, for documents) */
   int width, height; /* pixels, 0 when the header could not be read */
+  int pages;        /* PDF page count, 0 for anything else */
   char detail[8];   /* auto | low | high (wire hint, "" = auto) */
   char origin[32];  /* user | view_image | mcp (for display only) */
 } cst_attachment_t;
@@ -82,6 +85,7 @@ typedef struct {
   char model[128];
   char provider[32];
   char effort[16];      /* low | medium | high | max */
+  char mode[16];        /* coding | general (CST_DEFAULT_MODE) */
   cst_protocol_t protocol;
   int max_steps;        /* model calls per user turn */
   size_t tool_out_cap;  /* bytes of one tool output sent to the model */
@@ -114,6 +118,14 @@ typedef enum {
 const char *cst_approval_name(int a);
 int cst_approval_parse(const char *s, int *out);
 
+/* Working mode: how the system prompt frames the agent. "coding" (default)
+ * is code framed; "general" is for tasks that are not about a codebase
+ * (documents, data, research, everyday files). Orthogonal to plan mode. */
+#define CST_DEFAULT_MODE "coding"
+int cst_mode_valid(const char *m);
+/* Canonical name of m, or the default when m is unknown / empty. */
+const char *cst_mode_name(const char *m);
+
 typedef struct cst_session cst_session_t;
 typedef struct cst_agent cst_agent_t;
 
@@ -406,13 +418,14 @@ int cst_creds_resolve(const char *provider, char *out, size_t out_sz,
 /* Hidden console prompt (no echo). */
 int cst_creds_prompt_console(const char *provider, char *out, size_t out_sz);
 
-/* Last used provider, model per provider and effort, in
+/* Last used provider, model per provider, effort and working mode, in
  * <config>/settings.json. */
 #define CST_SETTINGS_MAX_MODELS 16
 #define CST_SETTINGS_MAX_TRUSTED 32
 typedef struct {
   char provider[32];
   char effort[16];
+  char mode[16];     /* coding | general, "" = not chosen yet */
   char approval[8]; /* "" = not chosen yet */
   int show_thinking; /* TUI: Thought blocks start expanded */
   /* workspaces whose .agent/plugins and .agent/mcp.json may run */
@@ -427,11 +440,13 @@ typedef struct {
 int cst_settings_load(cst_settings_t *s);
 int cst_settings_save(const cst_settings_t *s);
 const char *cst_settings_model(const cst_settings_t *s, const char *provider);
-/* Record cfg's provider, model and effort as the last used ones. */
+/* Record cfg's provider, model, effort and mode as the last used ones. */
 int cst_settings_remember(const cst_agent_config_t *cfg);
 /* Record an approval mode the user picked (CST_APPROVAL_*). */
 int cst_settings_remember_approval(int approval);
 int cst_settings_remember_thinking(int show_thinking);
+/* Record the working mode the user picked (coding | general). */
+int cst_settings_remember_mode(const char *mode);
 /* Workspace trust for project plugins / MCP servers (by real path). */
 int cst_settings_trusted(const char *workspace);
 int cst_settings_trust(const char *workspace, int on);