feat(02-01): wire FLYNN_ENV resolution into shared.ts with overlay-aware loadConfigSafe

- Add resolveOverlayPath() that maps FLYNN_ENV to {configDir}/{env}.yaml
- Update loadConfigSafe to pass overlay path through to loadConfig
- All CLI commands using loadConfigSafe() automatically get overlay support
- No FLYNN_ENV = exact same behavior as before (backward compatible)
- Full test suite passes (1087 tests, zero regressions)

src/cli/shared.ts

@@ -1,6 +1,6 @@
 import { loadConfig } from '../config/index.js';
 import type { Config } from '../config/index.js';
-import { resolve } from 'path';
+import { resolve, dirname, join } from 'path';
 import { homedir } from 'os';
 
 /** Get the config file path from env or default location. */
@@ -13,11 +13,25 @@ export function getDataDir(): string {
   return process.env.FLYNN_DATA_DIR ?? resolve(homedir(), '.local/share/flynn');
 }
 
+/**
+ * Resolve overlay config path from FLYNN_ENV.
+ * If FLYNN_ENV is set, returns {configDir}/{FLYNN_ENV}.yaml relative to the base config file.
+ * Returns undefined if FLYNN_ENV is not set.
+ * Does NOT check if the file exists — caller decides error handling.
+ */
+export function resolveOverlayPath(basePath: string): string | undefined {
+  const env = process.env.FLYNN_ENV;
+  if (!env) return undefined;
+  const configDir = dirname(basePath);
+  return join(configDir, `${env}.yaml`);
+}
+
 /** Load config without throwing. Returns { config } or { error }. */
 export function loadConfigSafe(configPath?: string): { config?: Config; error?: string } {
   const path = configPath ?? getConfigPath();
   try {
-    const config = loadConfig(path);
+    const overlayPath = resolveOverlayPath(path);
+    const config = loadConfig(path, overlayPath);
     return { config };
   } catch (error) {
     const message = error instanceof Error ? error.message : String(error);