v0.9.6: documents, downloads, PDFs and the phone's own files

Four tools and a document writer, plus the storage half of the phone: until
now the picker's answer (a content:// URI) reached the engine as a path, and
it said "cannot read" -- the app had no way to hand a phone's file over.

- md_to_html (new, src/tools/web.c + cst_web_md_document()): markdown in, one
  self-contained HTML document out -- doctype, utf-8, viewport, escaped
  title, a small stylesheet; no script, no external request. It is written
  into the workspace and delivered as a file card (for_user, so it never
  enters a request), the way deliver_file does. The default name comes from
  the source file's stem or, for pasted markdown, from the title with what a
  path cannot carry turned into '-'. It has a preview, so an approval prompt
  says "render x -> y.html" before anything is written.
- download (src/tools/web.c): an http(s) URL into a file in the workspace,
  streamed with a 128 MB ceiling; a redirect chain, the same CA handling and
  the same refusals as web_fetch, but the bytes stay on disk (a half file is
  removed on any failure, including HTTP >= 400). The name comes from the URL
  when the caller names only a directory.
- read_pdf (src/tools/fs_tools.c): the text layer and page count of a PDF,
  which read_file refuses because the bytes are binary -- and a PDF is
  exactly what arrives from a phone. Same extractor an attachment uses, its
  own 4 MiB input ceiling, 256 KiB of text by default with a note when cut.
- session_search (new, src/tools/session_tools.c): every stored session --
  this one and all the ones before it -- searched for a substring, reporting
  the session's id, message count and title, then the matching lines with
  their number and role. Case-insensitive by default, capped at 20 matches.
- device_info (same file, perm_name "device"): version, system/arch/CPU
  count, workspace, sandbox, model and permissions, config/temp/program
  directories, pid -- so the model stops guessing where it runs.
- Tools 3 and 4 come with tests/test_tools_ext.c (also in both build files):
  the document is a whole page, a script tag and a javascript: link do not
  survive, a hostile title stays text and cannot write outside the workspace,
  the preview names the file, read_pdf/download refuse what they must (no
  network in a test), session_search finds a fixture session and says so when
  nothing matches, device_info names the workspace. README's tool table grew
  five rows.

- Android storage: the picker (SAF) answers with a content:// URI, and the
  engine opens files by path, so the app now copies what it points at into
  its own directory first -- app/android/src/xyz/whcself/cstagent/
  AttachFile.java, called through a new app::materializeAttachment()
  (ToolProvider.h: one API for every platform; a desktop strips file:// and
  refuses content:// with a reason). The copy needs no permission at all: the
  grant travels with the URI.
- "All files access" (MANAGE_EXTERNAL_STORAGE) is the other half: it is what
  lets the engine read a path in shared storage that a task names rather than
  picks. Android has no runtime dialog for it, so main.cpp opens the one
  Settings screen once (AttachFile.java's requestAllFilesAccess, with the
  all-apps screen as a fallback) and never insists: without it everything
  works, one picked file at a time. On the NX789J the screen came up, the
  app-op reads back "allow", and a relaunch no longer asks.
- Composer.qml hands a content:// URI through untouched (it is not a path,
  and decoding it would corrupt it); AgentCore::attachFile() turns whatever
  the picker gave into a path first.

Desktop: build clean, make check 36/36 (the new test included, also under
ASan), --uitest all green, --selftest and cst-app-selftest OK.
The APK builds and installs (86 MB, release-signed) with the new Java class
in its dex.

Version 0.9.6.

Makefile

@@ -104,6 +104,7 @@ CORE_SRC = \
 	src/tools/fs_tools.c \
 	src/tools/agent_tools.c \
 	src/tools/web.c \
+	src/tools/session_tools.c \
 	src/tools/diff.c \
 	src/undo.c \
 	src/permissions.c \
@@ -146,6 +147,7 @@ TESTS = \
 	tests/test_shell \
 	tests/test_diff \
 	tests/test_tools_fs \
+	tests/test_tools_ext \
 	tests/test_registry \
 	tests/test_tool_meta \
 	tests/test_embed \