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.

src/subagent.c

@@ -40,7 +40,7 @@ static const struct {
      "runs commands as needed.",
      "", 0},
     {"explore",
-     "Fast read-only explorer: searches and reads the codebase to answer a "
+     "Fast read-only explorer: searches and reads the workspace to answer a "
      "question (where is X, how does Y work, which files do Z). Several can "
      "run at once.",
      "You only read. Search broadly first (grep, glob, list_dir), then read "
@@ -49,11 +49,11 @@ static const struct {
      "you looked for and did not find.",
      1},
     {"plan",
-     "Read-only planner: investigates the code involved and returns a "
+     "Read-only planner: investigates the material involved and returns a "
      "step-by-step implementation plan.",
-     "You only read. Investigate the code involved, then return a concrete "
-     "plan: the files to change and what to change in each, risks and edge "
-     "cases, and how to verify. Sketch code only where it removes ambiguity.",
+     "You only read. Investigate what is involved, then return a concrete "
+     "plan: what to produce or change and where, risks and edge cases, and "
+     "how to check the result. Sketch code only where it removes ambiguity.",
      1},
 };
 #define NBUILTIN ((int)(sizeof(BUILTIN) / sizeof(BUILTIN[0])))