Copilot re-auth prompted every session despite stored credential — expires:0, refresh == access

Description

After completing the GitHub Copilot device-code login successfully(the student package), opencode still
prompts to reauthenticate on every new session, even though opencode auth list shows
a credential is stored.

Steps to reproduce

1. opencode auth login → GitHub Copilot → GitHub.com → complete device code flow
2. opencode auth list confirms a credential exists
3. Close and reopen opencode (or start a new session) → prompted to reauth again

Observation

Contents of ~/.local/share/opencode/auth.json after a fresh login:

{
  "github-copilot": {
    "type": "oauth",
    "access": "gho_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
    "refresh": "gho_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
    "expires": 0
  }
}

Two things stand out that may or may not be related to the reauth prompts:

• expires is 0 right after a successful login
• access and refresh have identical values

Not sure if this is expected behavior for how Copilot's device flow issues tokens, or a
storage bug — flagging in case it's useful for reproducing the reauth issue.

Environment

• OpenCode version: [1.18.11]
• OS: Linux(ubuntu 24.04 LTS)
• Shell: fish
  -terminal-emulator: alacritty
• Auth: GitHub Copilot, device code flow, GitHub.com