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.

Makefile

@@ -96,6 +96,7 @@ CORE_SRC = \
 	src/keymap.c \
 	src/skills.c \
 	src/extensions.c src/plugins.c src/mcp.c src/subagent.c src/attach.c src/clipboard.c \
+	src/pdf_text.c \
 	src/session.c \
 	src/tools/tools.c \
 	src/tools/patch.c \
@@ -135,6 +136,7 @@ TESTS = \
 	tests/test_apply_patch \
 	tests/test_session_roundtrip \
 	tests/test_attach \
+	tests/test_pdf_text \
 	tests/test_shell \
 	tests/test_diff \
 	tests/test_tools_fs \
@@ -229,6 +231,9 @@ tests/test_mm_tool: tests/test_mm_tool.c src/mm_tool.c include/mm_tool.h $(TEST_
 tests/test_textsel: tests/test_textsel.c src/textsel.c $(TEST_OS_SRC) include/cst_textsel.h
 	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ tests/test_textsel.c src/textsel.c $(TEST_OS_SRC) $(TEST_SYS_LIBS)
 
+tests/test_pdf_text: tests/test_pdf_text.c src/pdf_text.c include/cst_pdf.h $(TEST_OS_SRC)
+	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ tests/test_pdf_text.c src/pdf_text.c $(TEST_OS_SRC) $(TEST_SYS_LIBS)
+
 tests/test_markdown: tests/test_markdown.c $(SD_DIR)/libsashidown.a
 	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ tests/test_markdown.c $(LDFLAGS) -lsashidown -lm