docs(agents): add Pi Durable page and update Pi docs for piDurable()

[eersnington]

• Add a Pi Durable page: what durable means, when to use it, tools and tasks, calling and streaming, c.pi, moving from pi(), upgrades and crashes, known limitations, and configuration
• Add Pi Durable to the Quickstart as a second harness tab, with E2B and Daytona examples
• Update the Pi page with when to use it, c.pi, and known limitations
• Cover upgrades and Pi Durable in Architecture, and add a background-agent pattern to Design Patterns
• Fix examples: registry.start() in workflows, local imports in the subagents and tenant-credentials examples, and a dead link in User Subscriptions
• Add Pi Durable snippets and their dependencies to the docs examples

[railway-app]

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