Handle provider subscription rate-limit retries

[ymichael]

Summary

• normalize Codex and Claude subscription rate-limit telemetry into a shared provider state
• add guarded, at-most-once continuation on the same provider conversation using agent-only Please continue.
• add the opt-in builtin provider-retry plugin with in-memory scheduling, account coordination, UI, SDK, and CLI controls
• enable it explicitly through Extensions → Plugins or bb plugin enable provider-retry
• allow plugins to be enabled or disabled directly from their settings detail page
• intentionally exclude retry persistence across server or daemon restarts

Safety

• continuation is allowed only for the latest terminal failed turn with no emitted output or possible side effect
• provider-owned retries remain authoritative while active
• reset changes, host disconnects, cancellation races, and realtime reconnection are handled explicitly
• credits and spend controls remain manual-retry cases

Validation

• 13 affected Turbo typecheck tasks pass
• app: 2,354 tests pass
• server: 1,330 tests pass
• agent runtime: 802 tests pass
• provider-retry plugin: 10 tests pass
• host-daemon contract: 49 tests pass
• CLI, SDK, plugin SDK, server contract, templates, and domain suites pass
• standalone provider-retry plugin build passes