Add Pi coding agent provider skeleton

[dersam]

Register :pi as a valid agent provider alongside :claude.

• Add Providers::Pi with Output class delegating to PiInvocation::Result
• Add PiInvocation with full lifecycle (start/run/complete/fail states)
• Build pi CLI command line: pi -p --mode json with model, system prompt,
  append system prompt, session, and custom command support
• Parse Pi's streaming JSON output (session, agent_end, turn_end,
  message_update types) to extract response, session ID, and usage stats
• Add comprehensive tests mirroring ClaudeInvocation test patterns

Pi's JSON streaming protocol differs from Claude Code's:

• Output flag: --mode json (vs --output-format stream-json)
• Non-interactive: -p (vs -p --verbose)
• Session resume: --session (vs --fork-session --resume )
• Result extraction from agent_end message (vs type: result)
• Usage stats from turn_end messages (vs modelUsage in result)

[dersam]

• 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
• Add Pi coding agent provider skeleton #830 👈 (View in Graphite)
• main

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