Fix self-hosted setup and Codex CLI JSONL handling

Dogfooding fizzy-popper with self-hosted Fizzy exposed a few setup/Codex rough edges.

## Problems

- `setup` assumes `https://app.fizzy.do` instead of using the API URL configured by the Fizzy CLI.
- README/setup-board examples still use board/column `--title`; current Fizzy CLI expects `--name`.
- Codex backend should send the prompt through stdin, set the working directory, and parse Codex JSONL down to the final agent message.
- Backend timeout/failure comments should stay concise and not include raw Codex JSONL transcripts.

## Expected Fix

- support `FIZZY_API_URL` and `fizzy config show --json` for self-hosted API URLs
- update setup docs for current Fizzy CLI flags
- update Codex invocation and JSONL parsing
- sanitize backend error output before posting comments

Draft patch: #4

Start by inspecting the setup flow and the README/setup-board examples, then trace the Codex backend invocation. Verify the Fizzy API URL and CLI flags, stdin and working-directory handling, final-message JSONL parsing, and concise sanitized failure comments; the work is done when the self-hosted setup and Codex paths meet all listed expectations.

Written by the indexing model from the issue text.