shell: absolute Unix socket arguments fail during realPath advisory scan

Summary

The V2 shell tool fails before executing commands that contain an absolute path to a live Unix-domain socket. The best-effort command-argument advisory scan surfaces Unknown: FileSystem.realPath (...) as a fatal tool error.

Environment

• opencode version: 0.0.0-next-16076
• OS: Darwin 25.5.0, arm64
• Terminal: WarpTerminal, TERM=xterm-256color, COLORTERM=truecolor
• Shell: /bin/zsh
• Install/channel: next, managed service healthy on the same version
• Active plugins: cmux-feed.js, cmux-session.js, voice.ts

Reproduction

1. Create a live Unix-domain socket:

   ruby -rsocket -e 's=UNIXServer.new(%q[/tmp/opencode-realpath-live.sock]); sleep 60'
   

2. While the socket is live, invoke the V2 shell tool with any command containing its absolute path:

   printf '%s\n' /tmp/opencode-realpath-live.sock
   

3. Observe that the shell command is not executed and the tool fails.

Expected Behavior

The shell command executes. Because command-argument path scanning is documented as best-effort and advisory, an unusual filesystem node should emit at most an external-directory warning and must not fail the command.

Actual Behavior

The shell tool fails before execution:

Unknown: FileSystem.realPath (/tmp/opencode-realpath-live.sock)

This reproduces consistently for a live Unix-domain socket. The identical printf command succeeds after the socket node disappears and emits only the normal external-directory advisory.

Additional Context

This was first encountered while sending a command to a local application over a Unix socket:

cargo run -q -p cutmanctl -- --socket /tmp/cutman-media.sock seek /tmp/cutman-fixture.mp4 2500 --base-revision 0

The failure appears to be in the advisory scanner rather than command execution:

• packages/core/src/tool/shell.ts tokenizes absolute command arguments and passes each through FSUtil.resolve in externalCommandDirectories.
• packages/util/src/fs-util.ts calls fs.realPath and recovers only from PlatformError with reason NotFound.
• On this system, realPath against a live Unix-domain socket returns an Unknown filesystem error, which escapes the supposedly advisory scan and aborts the shell tool.

A likely fix is to make argument-path canonicalization fully best-effort: preserve the lexically resolved path when realPath fails for any filesystem-node type, while retaining stricter resolution for actual shell working-directory authorization.

Start in packages/core/src/tool/shell.ts at externalCommandDirectories and follow its FSUtil.resolve calls into packages/util/src/fs-util.ts. Reproduce with a live Unix socket and the printf command, then verify that argument scanning remains advisory: the command executes and only the normal external-directory warning appears.