fix(cli): exit cleanly when no ChatGPT token is present

[robotlearning123]

Problem

The most common first-run state for a new user is installed + registered with the MCP client, but codex login / gpt2agent setup not run yet — so no token exists. The install flow does not require a token (it prints "No auth.json yet, run codex login"), so this state is normal after a one-line install + client restart.

In that state, the MCP client spawns gpt2agent run --stdio, which raised a bare RuntimeError out of BackendClient() inside build_server() and dumped a backend.py traceback into the client's MCP server logs:

Traceback (most recent call last):
  File ".../bin/gpt2agent", line 6, in <module>
    sys.exit(main())
  ...
  File ".../gpt2agent/backend.py", line 76, in _load_token_with_source
    raise RuntimeError(...)
RuntimeError: No ChatGPT token found — run `codex login` or `gpt2agent setup` (...)

The unauthenticated-HTTP-bind refusal path right below it already exits cleanly via SystemExit; the far more common missing-token path did not. This is a launch-quality inconsistency.

Fix

• Introduce TokenNotFoundError(RuntimeError) in backend.py and raise it (instead of bare RuntimeError) from _load_token_with_source(). It's a RuntimeError subclass, so every existing except RuntimeError handler and pytest.raises(RuntimeError, ...) assertion keeps working unchanged.
• Catch it in server.main() and exit 1 with the actionable one-line message via SystemExit — matching the HTTP-bind refusal path — instead of a traceback.

Before / after (real output)

# before:  HOME=<empty> gpt2agent run --stdio
Traceback (most recent call last): ... RuntimeError: No ChatGPT token found ...

# after:   HOME=<empty> gpt2agent run --stdio
No ChatGPT token found — run `codex login` or `gpt2agent setup` (checked .../.codex/auth.json and .../.gpt2agent/token.json)
$ echo $?
1

Server still boots normally when a token is present (verified: exit 0 on stdin close).

Tests

Adds tests/test_audit_2026_07_13_startup.py:

• TokenNotFoundError is a RuntimeError subclass and is raised on missing token.
• main() with run --stdio and no token raises SystemExit carrying the actionable message (no traceback).

Full suite: 325 passed, 9 skipped; ruff check clean.

🤖 Generated with Claude Code

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 54966cab-e083-42a3-b6d6-f540eaf47160

📥 Commits

Reviewing files that changed from the base of the PR and between 04e3d93 and f85f435.

📒 Files selected for processing (3)

• gpt2agent/backend.py
• gpt2agent/server.py
• tests/test_audit_2026_07_13_startup.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/clean-token-startup-error

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