## Why

BooTerm currently provides basic terminal panes but lacks structured process lifecycle awareness. AI agents don't know when background tasks complete, what processes are running, or which agent spawned which terminal. Adding PTY exit notifications, rich session metadata, sandbox circuit breaker, and X-Agent-Flags header support gives BooTerm production-grade terminal management.

## What Changes

- Add PTY exit notifications from opencode-pty (`<pty_exited>` structured payload with exit code, last output line)
- Add session metadata for PTY panes (title, description, parent agent, task ID)
- Add pattern-based log search with offset/limit pagination
- Add per-session timeouts for runaway process protection
- Add sandbox circuit breaker from open-swe (consecutive failures → abort thread)
- Add sandbox lifecycle management (4 cases: cached, creating, missing, metadata-reconnect)
- Add X-Agent-Flags header support in llama-sidecar (per-agent LLM parameter overrides)
- Add GitHub proxy pattern for authenticated git operations in sandbox

## Capabilities

### New Capabilities
- `pty-exit-notifications`: Structured exit payload on process completion
- `pty-session-metadata`: Title, description, parent agent per terminal pane
- `pty-log-search`: Pattern-based PTY buffer search with pagination
- `pty-timeouts`: Per-session runtime limits for process protection
- `sandbox-circuit-breaker`: Consecutive failure detection and thread abort
- `sandbox-lifecycle`: Formal sandbox lifecycle management (4 cases)
- `agent-flags-override`: Per-agent LLM parameter overrides via X-Agent-Flags
- `sandbox-github-proxy`: Authenticated git operations without storing tokens
