Fix self-hosted setup and Codex CLI JSONL handling

[joshyorko]

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