fix(opencode): preserve user request for skill slash commands

[BrickerP]

Issue for this PR

Closes #40463

Related / previously reported: #26185, #27686, #28241 (closed as dupes / stale)

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Skill slash commands (/skill-name user request here) were registered as normal commands whose template is the raw SKILL.md body. SessionPrompt.command then ran $ARGUMENTS / $N substitution against that body.

That caused two failure modes:

1. Skill docs that mention $ARGUMENTS or $1 (common in examples) swallowed the real user args into those example spots.
2. Skills without placeholders only appended the user text after a long skill body, so models often ignored the request and asked "what do you mean?".

Fix: when cmd.source === "skill", do not substitute placeholders inside the skill body. Wrap it as:

Follow the skill instructions below for skill "<name>".

<skill_content name="<name>">
...raw skill body, placeholders left literal...
</skill_content>

## User request
<user arguments or a clear empty-request note>

Regular commands and MCP prompts keep the existing $ARGUMENTS / $N / append behavior.

Also set skill command hints to ["$ARGUMENTS"] so the slash UI can show that trailing input is supported.

How did you verify your code works?

cd packages/opencode
bun test test/session/prompt.test.ts -t "skill slash command preserves|unknown command|command ! expansion"

Result: 3 pass, 0 fail (includes new regression test that asserts $ARGUMENTS/$1 in skill docs stay literal and the user request appears under ## User request).

Screenshots / recordings

N/A — server-side prompt construction only.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found some related PRs but no duplicates of PR #40472:

Related PRs (not duplicates):

1. fix(opencode): include skill files when invoking via slash command #35522 - fix(opencode): include skill files when invoking via slash command - Related to skill slash command handling
2. fix(session): hide skill template text from chat and terminal UIs #28239 - fix(session): hide skill template text from chat and terminal UIs - Related to skill template display/processing
3. fix(tui): insert skill name without wiping existing prompt text #27632 - fix(tui): insert skill name without wiping existing prompt text - Related to skill command interaction in TUI

These are addressing related issues around skill commands but are distinct from PR #40472's specific fix about preserving user requests and preventing placeholder substitution within skill bodies.

No duplicate PRs found.