feat: mirror mode for MCP result-artifact rules

[roackb2]

Why

Result-artifact capture normally replaces captured values with compact references. That is appropriate for context compaction, but it breaks stateless MCP servers where each tool call must receive the full document returned by the previous call.

Without a non-replacing capture mode, hosts must disable result artifacts and manually inspect raw tool-result JSON to recover the final document.

What

A manual result-artifact rule can now declare mode: 'mirror':

resultArtifacts: [{
  toolName: 'create-deck',
  path: 'structuredContent.result.source',
  mode: 'mirror',
  kind: 'source',
  setCurrent: true,
}]

Mirror mode:

• persists the artifact with the normal metadata and setCurrent behavior;
• leaves the declared value inline for downstream stateless tool calls;
• ignores replacePaths, because no result path is replaced;
• lets the host read the latest result through engine.artifacts.current(sessionId);
• takes precedence over a combined auto-capture pass: the mirrored path and its descendants are excluded from auto replacement.

The default 'replace' behavior is unchanged.

Safety

This captures tool results; it does not expose model-generated private output. The host still decides which artifact content becomes user-visible.

Verification

• Focused MCP host-extension tests: 14 passed
• Unit: 603 passed
• Integration: 285 passed
• yarn lint
• yarn build