feat(llm9p): Implement clone-based session architecture

Replace per-fid session model with Plan 9 clone pattern:
- Reading /n/llm/new creates a session and returns its ID
- Each session gets its own directory: /n/llm/<id>/
- Per-session files: ask, ctl, model, system, thinking, context, metrics
- AskWithRequest method for stateless CSP-style LLM calls
- Session settings (model, temperature, thinking) are per-session
- Remove old ask.go, context.go in favor of session-scoped files

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

feat(llm9p): Add per-fid session isolation and prefill support

- Add SessionManager for per-fid conversation isolation
- Each 9P fid now gets its own conversation history
- Add FidAwareFile interface for files needing fid context
- Add /n/llm/prefill file for assistant response prefill
- Prefill helps keep model in character (e.g., "[Veltro]")
- Update ask, new, context files to use session manager
- Fix context contamination between parent and subagent

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

feat(llm): Add extended thinking support and usage tracking

- Add thinking token control via /n/llm/thinking file (max/off/number)
- CLI backend sets MAX_THINKING_TOKENS env var for Claude CLI
- Default to max thinking (31999 tokens) for CLI backend
- Add /n/llm/usage file for token usage monitoring
- Add /n/llm/compact file for conversation summarization
- Extend Backend interface with ThinkingTokens, TotalTokens, ContextLimit, Compact
- Add true streaming support for CLI backend with line-by-line output
- Update example file with thinking documentation

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