fix(cli): add --flat to KNOWN_FLAGS so spawn list --flat works (#3137)

The --flat flag was documented in help output and used by `spawn list`
but missing from KNOWN_FLAGS, causing an "Unknown flag" error.

Agent: ux-engineer

Co-authored-by: B <6723574+louisgv@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

packages/cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@openrouter/spawn",
-  "version": "0.30.3",
+  "version": "0.30.4",
   "type": "module",
   "bin": {
     "spawn": "cli.js"

packages/cli/src/__tests__/unknown-flags.test.ts

@@ -228,6 +228,7 @@ describe("KNOWN_FLAGS completeness", () => {
       "--config",
       "--steps",
       "--fast",
+      "--flat",
       "--user",
       "-u",
       "--yes",

packages/cli/src/flags.ts

@@ -36,6 +36,7 @@ export const KNOWN_FLAGS = new Set([
   "--config",
   "--steps",
   "--fast",
+  "--flat",
   "--user",
   "-u",
   "--yes",