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';

src/utils/opencode-cli-resolver.ts

@@ -0,0 +1,95 @@
+/**
+ * @fileoverview Resolve the OpenCode CLI binary across common install paths.
+ *
+ * Mirrors claude-cli-resolver.ts pattern. Finds the `opencode` binary
+ * and provides an augmented PATH string for tmux sessions.
+ *
+ * @module utils/opencode-cli-resolver
+ */
+
+import { execSync } from 'node:child_process';
+import { existsSync } from 'node:fs';
+import { delimiter, dirname, join } from 'node:path';
+import { homedir } from 'node:os';
+
+/** Timeout for exec commands (5 seconds) */
+const EXEC_TIMEOUT_MS = 5000;
+
+/** Common directories where the OpenCode CLI binary may be installed */
+const OPENCODE_SEARCH_DIRS = [
+  join(homedir(), '.opencode', 'bin'),    // Default install location
+  join(homedir(), '.local', 'bin'),       // Alternative install location
+  '/usr/local/bin',                       // Homebrew / system
+  join(homedir(), 'go', 'bin'),           // Go install
+  join(homedir(), '.bun', 'bin'),         // Bun global
+  join(homedir(), '.npm-global', 'bin'),  // npm global
+  join(homedir(), 'bin'),                 // User bin
+];
+
+/** Cached directory containing the opencode binary (empty string = searched but not found) */
+let _openCodeDir: string | null = null;
+
+/**
+ * Finds the directory containing the `opencode` binary.
+ * Checks `which opencode` first, then falls back to common install locations.
+ * Result is cached for subsequent calls.
+ *
+ * @returns Directory path, or null if not found
+ */
+export function resolveOpenCodeDir(): string | null {
+  if (_openCodeDir !== null) return _openCodeDir || null;
+
+  // Try `which` first (respects current PATH)
+  try {
+    const result = execSync('which opencode', { encoding: 'utf-8', timeout: EXEC_TIMEOUT_MS }).trim();
+    if (result && existsSync(result)) {
+      _openCodeDir = dirname(result);
+      return _openCodeDir;
+    }
+  } catch {
+    // OpenCode not in PATH, will check common locations
+  }
+
+  // Fallback: check common installation directories
+  for (const dir of OPENCODE_SEARCH_DIRS) {
+    if (existsSync(join(dir, 'opencode'))) {
+      _openCodeDir = dir;
+      return _openCodeDir;
+    }
+  }
+
+  _openCodeDir = '';  // mark as searched, not found
+  return null;
+}
+
+/**
+ * Check if OpenCode CLI is available on the system.
+ */
+export function isOpenCodeAvailable(): boolean {
+  return resolveOpenCodeDir() !== null;
+}
+
+/**
+ * Returns a PATH string that includes the directory containing `opencode`.
+ *
+ * Finds the opencode binary (via `which` or common install locations), then
+ * prepends its directory to the current PATH if not already present.
+ * Result is cached for subsequent calls.
+ */
+export function getOpenCodeAugmentedPath(): string {
+  const currentPath = process.env.PATH || '';
+  const dir = resolveOpenCodeDir();
+
+  if (dir && !currentPath.split(delimiter).includes(dir)) {
+    return `${dir}${delimiter}${currentPath}`;
+  }
+
+  return currentPath;
+}
+
+/**
+ * Reset cached resolution (for testing).
+ */
+export function resetOpenCodeCache(): void {
+  _openCodeDir = null;
+}