Implement the documented --all-accounts usage behavior

[tsouth89]

Problem

The CLI defines and documents codexbar usage --all-accounts, and the top-level command forwards the flag, but UsageCommand does not retain it. Execution still fetches only the active account, so the output is identical to omitting the flag.

Where

• rust/src/cli/mod.rs
• rust/src/cli/usage.rs
• account selection in rust/src/core/provider.rs

Expected behavior

• Carry all_accounts into execution.
• Enumerate configured accounts for providers that support account directories/tokens.
• Identify each result consistently in text and JSON output.
• Keep the existing single-active-account behavior when the flag is absent.
• Use bounded concurrency and isolate one account failure from the others.
• Document the JSON compatibility decision.

Tests

Cover multiple Codex/Claude accounts, one failing account, providers without account support, and output with/without the flag.