feat(shell): add /sh shell command execution and persistent shell mode

[yuanguangshan]

Add WeChat-accessible shell command interface via /sh and /$ commands.

Features

/sh — Single command execution

• Execute read-only shell commands directly from WeChat
• Usage: /sh ls -la, /sh cat README.md
• /$ <cmd> is an alias

/sh — Persistent shell mode

• Enter interactive shell mode with persistent working directory
• Type commands without prefix (like a real terminal)
• Exit with /q
• Prompt displays current path: /path/to/dir:#
• Auto-runs ls after each cd

Security — defense in depth

• Command whitelist: only safe read-only commands allowed (ls, pwd, cd, cat, head, tail, grep, find, wc, du, df, file, stat, date, echo, basename, dirname, realpath, readlink, which, tree, nl, sort, uniq, cut, awk, sed, tr, xargs)
• Operator blacklist: blocks |, >, &&, ||, ;, `````, $(
• Symlink escape protection: resolves symlinks before cd
• Sandbox support: optional baseDir restriction per session
• File size guard: cat files > 50KB are rejected with tips

UX polish

• Shortcut aliases: ll → ls -lh, .. → cd .., ... → cd ../..
• Auto-enhance ls with -C (column layout) flag
• ANSI escape code stripping for clean WeChat display
• Output truncation at 4000 chars (WeChat message limit)
• Markdown code block wrapping for monospace rendering

Files changed

• messaging/handler.go: shell mode state, routing, handleShell, enterShellMode, handleShellWithState, cleanANSI, helpers
• docs/shell-mode-guide.md: full user guide with examples