feat: Add system prompt file for persistent persona configuration

- Add system file (read/write) to set system prompt
- System prompt persists across conversation resets
- Add SystemPrompt() and SetSystemPrompt() to Backend interface
- Update both API and CLI clients to support dedicated system prompt
- Update documentation and examples

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

feat: Add stream/ask file to trigger streaming requests

- Add StreamAskFile to start streaming via write to stream/ask
- Read chunks from stream/chunk as they arrive
- Update documentation with streaming examples and verified tests
- Update _example file with correct streaming instructions

Streaming workflow:
1. Write prompt to stream/ask to start streaming
2. Read from stream/chunk to get chunks (blocks until available)
3. EOF returned when stream completes

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

feat: Initial implementation of llm9p - LLM as 9P filesystem

Exposes Claude as a 9P filesystem, enabling interaction through
standard file operations:

- ask: write prompt, read response (shim pattern)
- model: read/write current model name
- temperature: read/write sampling temperature
- tokens: read-only token count from last response
- new: write to reset conversation
- context: read JSON history, write to add system message
- _example: usage documentation
- stream/chunk: blocking read for streaming responses

Includes:
- Full 9P2000 protocol implementation (stdlib only)
- Anthropic SDK integration with conversation state
- Streaming support

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>