fix(config): honor frontmatter prompt when agent .md body is empty

[rogerdigital]

Issue for this PR

Closes #26434

Type of change

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

What does this PR do?

When an agent .md file declares prompt: in YAML frontmatter but has an empty markdown body, config/agent.ts unconditionally overwrites the frontmatter value with the empty body string. This causes the agent to silently fall back to the default system prompt with no warning.

The fix changes the precedence logic in both load() and loadMode():

1. Body takes precedence only when non-empty — if body is empty, frontmatter prompt: is used instead. This is fully backward-compatible since all existing agents that use the body convention have non-empty bodies.

2. Warns when both are present — if an agent has both a non-empty body and frontmatter prompt:, a warning is logged clarifying that body wins. This catches ambiguous configs at load time.

3. Applies ConfigVariable.substitute to agent/mode .md prompts — {env:VAR} and {file:path} placeholders now resolve in .md agent prompts, consistent with how they already work in opencode.json agent configs. Previously these stayed as literal strings in .md files.

4. Runtime warning when custom agent prompt is empty — in llm.ts, logs a warning if a non-native agent's prompt resolves to empty before falling back to the provider default. This catches any remaining silent-empty-prompt path.

How did you verify your code works?

• Added 4 test cases in test/config/config.test.ts:
  • Frontmatter-only prompt (empty body) → uses frontmatter value
  • Both body and frontmatter → body wins
  • {env:VAR} substitution in body
  • {file:path} substitution in frontmatter prompt
• All 85 config tests pass, all 40 agent tests pass
• Typecheck clean on modified files (pre-existing failures in unrelated http-recorder package)

Screenshots / recordings

N/A — no UI changes.

Checklist

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