fix(queue): deliver a queued slash command with context as a prompt

OpenCode's command route accepts file parts only. The server queue was
attaching captured context as text parts to POST /session/:id/command, so
a slash command queued with a comment, quote, or PR diff was rejected with
400 and retried forever.

A command queued without context still takes the command route with its
files. One queued with context now takes the prompt route the way the
composer does: the command's template is expanded with its arguments, a
skill keeps its text and gets the explicit skill-invocation instruction,
and the context rides along.

Claude-Session: https://claude.ai/code/session_01VqV56Hez25hTxXH4ipJfzH

packages/web/server/lib/message-queue/DOCUMENTATION.md

@@ -95,13 +95,19 @@ persisted "sending" flag would strand a message forever.
    the tick re-arms with backoff.
 5. The head is marked in flight (broadcast), then sent:
    - text starting with `/` that names a command in OpenCode's `/command`
-     list (skills included) goes to `POST /session/:id/command` with the
-     captured model, agent, variant, and file parts;
+     list (skills included) and carries no captured context goes to
+     `POST /session/:id/command` with the captured model, agent, variant, and
+     file parts. That route accepts file parts only, so a command queued
+     **with** context takes the prompt route instead, the same rule the
+     composer applies: the command's template is expanded with its arguments
+     (`$ARGUMENTS`, `$1..$N`, or appended), a skill keeps its `/name args` text
+     and gets an explicit "the user invoked this skill" synthetic part after
+     the context;
    - otherwise `POST /session/:id/prompt_async` with the parts in the same
-     order a UI send uses: text, files, the captured context, pending project
-     knowledge (`sessionKnowledgeRuntime.resolvePendingForSession`, synthetic,
-     recorded as delivered only after the prompt is accepted), then the agent
-     mention. The command path sends files and captured context as `parts`.
+     order a UI send uses: text, files, the captured context, the skill
+     invocation when there is one, pending project knowledge
+     (`sessionKnowledgeRuntime.resolvePendingForSession`, synthetic, recorded
+     as delivered only after the prompt is accepted), then the agent mention.
    Success removes the item, persists, broadcasts, and marks the user
    message sent for notifications. Failure keeps the item, backs off
    2 s → 60 s (doubling per consecutive failure of that item), and re-arms.