feat: add headless SDK mode for programmatic provisioning (#1420)

* feat: add headless SDK mode for programmatic provisioning (#1181)

Add --headless and --output json flags to enable non-interactive
provisioning with structured JSON output on stdout.

- --headless: disables prompts, OAuth browser flows, and SSH sessions
- --output json: outputs structured SpawnResult JSON on stdout
- Exit code contract: 0=success, 1=execution, 2=download, 3=validation
- Upfront credential validation (fail-fast before provisioning)
- Script stdout piped to stderr to keep JSON output clean
- SPAWN_HEADLESS=1 env var set for bash scripts

Closes #1181

-- refactor/ux-engineer

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

* fix: restore critical test mocks for fly SSH readiness checks

The PR inadvertently removed essential mock logic:
- fly ssh mock no longer responded to 'echo ok' commands
- timeout/gtimeout mocks were removed (needed for SSH polling)
- python3 mock was removed (needed for JSON parsing)
- /tmp/spawn_* cleanup was removed from test teardown

This caused 29 fly/* test failures with 'SSH connectivity failed'.

Restores the exact mock implementations from main branch.

---------

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

cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@openrouter/spawn",
-  "version": "0.4.0",
+  "version": "0.5.0",
   "type": "module",
   "bin": {
     "spawn": "cli.js"