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/types.ts

@@ -113,6 +113,23 @@ export interface LifecycleEntry {
  */
 export type ClaudeMode = 'dangerously-skip-permissions' | 'normal' | 'allowedTools';
 
+/** Session mode: which CLI backend a session runs */
+export type SessionMode = 'claude' | 'shell' | 'opencode';
+
+/** OpenCode session configuration */
+export interface OpenCodeConfig {
+  /** Model identifier (e.g., "anthropic/claude-sonnet-4-5", "openai/gpt-5.2", "ollama/codellama") */
+  model?: string;
+  /** Whether to auto-allow all tool executions (sets permission.* = allow) */
+  autoAllowTools?: boolean;
+  /** Session ID to continue from */
+  continueSession?: string;
+  /** Whether to fork when continuing (branch the conversation) */
+  forkSession?: boolean;
+  /** Custom inline config JSON (passed via OPENCODE_CONFIG_CONTENT) */
+  configContent?: string;
+}
+
 /**
  * Configuration for creating a new session
  */
@@ -158,8 +175,8 @@ export interface SessionState {
   lastActivityAt: number;
   /** Session display name */
   name?: string;
-  /** Session mode: 'claude' or 'shell' */
-  mode?: 'claude' | 'shell';
+  /** Session mode */
+  mode?: SessionMode;
   /** Auto-clear enabled */
   autoClearEnabled?: boolean;
   /** Auto-clear token threshold */
@@ -208,6 +225,8 @@ export interface SessionState {
   cliAccountType?: string;
   /** Latest CLI version available (parsed from version check) */
   cliLatestVersion?: string;
+  /** OpenCode-specific configuration (only for mode === 'opencode') */
+  openCodeConfig?: OpenCodeConfig;
 }
 
 // ========== Global Stats Types ==========