chore: remove dead code and unused exports (-159 lines)

Remove confirmed-unused code found via team-based codebase analysis:
- Dead interfaces: SessionResponse (types.ts), PlannerResult (plan-orchestrator.ts)
- Dead functions: withTimeout (session.ts), getOpenCodeAugmentedPath, resetOpenCodeCache (opencode-cli-resolver.ts)
- Dead config constants: MAX_RUN_SUMMARY_EVENTS, TRIM_RUN_SUMMARY_TO, MAX_MESSAGES_PER_CHANNEL (buffer-limits.ts), MAX_TRACKED_AGENTS, MAX_SUBAGENT_ACTIVITY_PER_AGENT, MAX_TOOL_RESULTS_PER_AGENT, COMPLETED_TODO_TTL_MS (map-limits.ts)
- Dead legacy code: idleTimer property + clearIdleTimer() no-op method + 6 call sites (respawn-controller.ts)
- Stale barrel re-exports: createAnsiPatternFull/Simple, validateTokenCounts/Cost, stripAnsi, normalizePhrase, getOpenCodeAugmentedPath (utils/index.ts)
- Un-exported internal-only symbols: ErrorMessages (types.ts), destroyRalphLoop (ralph-loop.ts)

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

src/utils/opencode-cli-resolver.ts

@@ -9,7 +9,7 @@
 
 import { execSync } from 'node:child_process';
 import { existsSync } from 'node:fs';
-import { delimiter, dirname, join } from 'node:path';
+import { dirname, join } from 'node:path';
 import { homedir } from 'node:os';
 
 /** Timeout for exec commands (5 seconds) */
@@ -71,28 +71,3 @@ export function resolveOpenCodeDir(): string | null {
 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;
-}