feat: OpenCode integration Phases 1-3 — types, CLI resolver, tmux spawn

Phase 1: Type system
- Add SessionMode = 'claude' | 'shell' | 'opencode' to types.ts (single source of truth)
- Add OpenCodeConfig interface (model, autoAllowTools, continueSession, etc.)
- Add openCodeConfig to SessionState
- Import SessionMode in session.ts, mux-interface.ts (remove duplicates)
- Refactor createSession/respawnPane to options objects (CreateSessionOptions, RespawnPaneOptions)

Phase 2: CLI resolver
- New opencode-cli-resolver.ts mirroring claude-cli-resolver.ts
- Searches ~/.opencode/bin, ~/.local/bin, /usr/local/bin, ~/go/bin, etc.
- Exported via utils/index.ts

Phase 3: Tmux spawn
- Extract buildSpawnCommand() shared helper (eliminates duplication)
- Add buildOpenCodeCommand() for opencode CLI flags
- Add setOpenCodeEnvVars() — API keys via tmux setenv (not visible in ps)
- Add setOpenCodeConfigContent() — JSON config via tmux setenv (prevents shell injection)
- Gate _processExpensiveParsers() for opencode mode (skip Claude-specific parsers)
- OpenCode-specific TUI ready detection (timeout-based, no ❯ prompt scanning)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/utils/index.ts

@@ -33,3 +33,4 @@ export {
 export { assertNever } from './type-safety.js';
 export { wrapWithNice } from './nice-wrapper.js';
 export { findClaudeDir, getAugmentedPath } from './claude-cli-resolver.js';
+export { resolveOpenCodeDir, isOpenCodeAvailable, getOpenCodeAugmentedPath } from './opencode-cli-resolver.js';