Add feature of designating the default account.

[FidesTherese]

Summary

This PR adds manual default-account selection for OpenAI multi-account authentication.
Users can now select an existing account with:
multiauth -d user@example.com
The selected account is preferred for each OpenAI session until it reaches a rate limit. The plugin then automatically switches and rebinds the session to an eligible fallback account.

Changes

• Added the multiauth command with -d and --default options.
• Added optional defaultAccountIndex persistence to the existing version-1 account schema.
• Added trimmed, case-insensitive email matching.
• Rejects unknown or ambiguous email addresses without modifying storage.
• Prefers the configured default on the first observable OpenAI request for each session and plugin process.
• Preserves existing behavior when no default account is configured.
• Persists rate-limit cooldowns before retrying.
• Rebinds sessions to fallback accounts after a 429 response.
• Keeps subsequent requests on the fallback account for the current session.
• Supports numeric and HTTP-date Retry-After headers.
• Clears or adjusts the default index when accounts are removed.
• Preserves the existing installer executable while adding the multiauth package binary.
• Replaced the POSIX-only build copy command with a cross-platform Node.js implementation.
• Updated user, configuration, testing, changelog, and architecture documentation.

Runtime Behavior

OpenCode does not expose a session.selected hook. Therefore, the first observable OpenAI request is used as the session initialization boundary.
Non-OpenAI providers are unaffected.

Compatibility

• Existing version-1 account files remain valid.
• No storage migration is required.
• Invalid or out-of-range default indexes are treated as unconfigured.
• No new runtime dependency was introduced.

Verification

• npm test: 304 tests passed.
• npm run typecheck: passed.
• npm run build: passed on Windows.
• npm pack --dry-run: confirmed the CLI and OAuth assets are included.
• multiauth --help: verified through PowerShell and CMD.
• Local OpenCode plugin loading: verified with opencode debug config.