fix(workspace-template): include auth_headers() on /registry/register POST

The register call was missing headers=auth_headers(), so workspaces that
already have a persisted token (i.e. every restart after the first boot)
were sending an unauthenticated request. The platform's register handler
returns 401 for requests missing a valid bearer token once a token has
been issued, causing re-registration to fail on every restart.

Import auth_headers at the module level (alongside the existing save_token
inline import) and pass it to the httpx POST. auth_headers() returns {}
when no token is on file yet (first boot), so there is no regression for
fresh workspaces — the platform still issues a token on the 200 response
and save_token() persists it for all subsequent restarts.

Closes #215

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

workspace-template/main.py

@@ -30,6 +30,7 @@ from initial_prompt import (
     mark_initial_prompt_attempted,
     resolve_initial_prompt_marker,
 )
+from platform_auth import auth_headers
 
 
 def get_machine_ip() -> str:  # pragma: no cover
@@ -195,6 +196,7 @@ async def main():  # pragma: no cover
                     "url": workspace_url,
                     "agent_card": agent_card_dict,
                 },
+                headers=auth_headers(),
             )
             print(f"Registered with platform: {resp.status_code}")
             # Phase 30.1 — capture the auth token issued at first register.