refactor(orchestrator): extract shared output parsing helper

## Context

~40 lines of CLI tool matching + parser dispatch are duplicated between `drain_output_events()` and `poll_agent_exits()`.

## Proposed fix

Extract `parse_agent_output(cli_tool, line, session_id, model) -> Option<CompletionEvent>` helper.

Refs terraphim/terraphim-ai#538 (Gitea), Refs #523