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.

README.md

@@ -300,6 +300,8 @@ cst-agent --headless --prompt 'List files'
 | `delete_file` / `move_file` | 写 | 删除 / 移动（重命名）文件 |
 | `run_shell` | shell | 非交互命令，超时整组进程被杀，Esc 可中断 |
 | `web_fetch` | 网络 | 抓取 URL，HTML 转纯文本 |
+| `download` | 网络 | 把 URL 抓成工作区里的文件（≤128 MB，超限删掉残file）；字节落在磁盘上，不进模型上下文 |
+| `read_pdf` | 读 | 读 PDF 的文本层（`read_file` 因二进制拒收的缺口），带页数，文本默认截到 256 KiB |
 | `clipboard_read` | 读 | 读系统剪贴板里的文本（`max_bytes` 默认 64 KiB，超出截断并说明） |
 | `clipboard_write` | 写 | 把文本放进系统剪贴板（按写类权限确认；本机走 pbcopy / wl-copy / xclip / Win32 API） |
 | `todo_write` | 元 | 维护任务清单（TUI 右侧面板，Ctrl+T 开关） |
@@ -307,6 +309,9 @@ cst-agent --headless --prompt 'List files'
 | `skill` | 元 | 加载一个 skill 的说明（`SKILL.md`）或其中的文件；有 skill 时才出现 |
 | `task` | 子 agent | 把独立任务交给子 agent（`general` / `explore` / `plan` / 自定义角色），只返回最终报告 |
 | `deliver_file` | 交付 | 把工作区里的成品文件复制进会话库并交给用户（任意字节，≤64 MB）：前端画成文件卡，可打开 / 分享；带 `for_user` 标记，永不进模型上下文 |
+| `md_to_html` | 文档 | 把 markdown（文本或工作区 `.md`）渲染成一份自包含 HTML 文档写进工作区并交付成文件卡：utf-8、标题、内联样式，无脚本无外链 |
+| `session_search` | 读 | 在过去和当前的所有会话转录里搜子串，给会话 id、条数、标题和匹配行（行号 + 角色 + 上下文） |
+| `device_info` | 读 | 报告运行环境：系统 / 架构 / CPU 数、工作区、沙箱、模型与审批模式、config / temp / 程序目录、pid（审批策略名 `device`） |
 
 插件工具和 MCP 工具（`mcp__<server>__<tool>`）在运行时注册，和内置工具走同一套审批与 `/tools` 开关。
 同一步里的多个只读调用（读文件、搜索、只读子 agent）会并行执行（最多 4 个）。