Add opt-in proactive auto-switch policy

[txhno]

Summary

• add an opt-in auto policy to config auto while keeping threshold as the default
• rank accounts by perishable safe capacity so healthy accounts with earlier reset pressure are burned first
• document the new policy and cover proactive switching behavior with CLI, registry, and auto-switch tests

Testing

• HOME=/tmp/codex-auth-verify-home zig build test
• HOME=/tmp/codex-auth-verify-home zig build run -- config auto --policy auto --5h 12 --weekly 8

Closes #41

Note

Add opt-in auto policy for proactive account switching in codex-auth

• Introduces a new AutoSwitchPolicy enum (threshold | auto) stored in the registry's AutoSwitchConfig, defaulting to threshold to preserve existing behavior.
• Under the auto policy, candidate accounts are scored using remaining usage headroom, time-to-reset, and burn rate rather than simple threshold crossing; the best-scoring candidate is selected proactively.
• Adds --policy <threshold|auto> to the config auto CLI command, with updated help, usage, and examples sections.
• Both the daemon loop and foreground switch path in auto.zig branch on the configured policy, with auto-policy candidates refreshed and validated via dedicated upkeep functions before switching.
• Status output now includes the active policy name alongside existing threshold values.

^{Macroscope summarized 29932a0.}

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commit: 29932a0