Add Pi message parsing infrastructure

[dersam]

Parse Pi's streaming JSON protocol into structured message objects,
mirroring the Claude provider's message architecture.

Message types implemented:

• SessionMessage: Initial session metadata (id, version, cwd)
• AgentStartMessage: Agent execution has begun
• AgentEndMessage: Agent execution complete with full conversation;
  extracts final response, model, and usage from last assistant message
• TurnStartMessage: New request/response cycle started
• TurnEndMessage: Turn complete with usage stats per model
• MessageStartMessage: User or assistant message starting
• MessageUpdateMessage: Streaming deltas (text_delta, text_start/end,
  toolcall_start/delta/end) with format() for progress display
• MessageEndMessage: Message complete with final content and usage
• ToolExecutionStartMessage / ToolExecutionEndMessage: Tool execution
• UnknownMessage: Catch-all for unrecognized types

The Message base class provides a from_json/from_hash factory that
dispatches to the appropriate subclass. PiInvocation now uses the
Message classes instead of raw hash parsing.

91 new tests (119 total Pi provider tests), full suite passes (985).

[dersam]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Add Pi provider documentation and examples #834
• Add Pi progress display and tool use formatting #833
• Add Pi invocation execution with fixture-based integration tests #832
• Add Pi message parsing infrastructure #831 👈 (View in Graphite)
• Add Pi coding agent provider skeleton #830
• main

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