Adds internal/provider/subprocess — a provider.Provider that spawns CLI
agents (claude, gemini, vibe) as subprocesses and streams their output.
- FormatParser interface + three parsers for claude-stream-json,
gemini-stream-json, and vibe-streaming formats; fixtures captured from
real binaries
- subprocessStream: pull-based stream.Stream over subprocess stdout with
bounded stderr capture (8KB) and guarded reap() to prevent double-Wait
- DiscoverCLIAgents: parallel PATH scan with 10s timeout, stable ordering
- Provider: only the last user message is passed as --prompt; all other
request fields (history, tools, system prompt) are intentionally ignored
(see package doc)
- main.go: discover and register CLI arms at startup; TODO(P0c) for
tier-based routing to enforce preference order explicitly
