docs: add repl-playground example for the durable REPL

[ben-reitz]

Adds examples/repl-playground, a runnable browser and HTTP playground for the durable REPL.

• Each Durable Object hosts a real Workspace with fake, deterministic grants (CRM, billing, metrics, logs, team, notify, fs, allowlisted fetch) seeded with an incident to investigate.
• The browser UI lets you run cells and view committed history, recorded effects, and live call counters, so you can watch replay make zero live calls. It also has a sidebar that documents each grant.
• scripts/play.mjs is a small CLI for the same endpoints.
• Auth is a bearer token, or a verified Cloudflare Access JWT if ACCESS_TEAM/ACCESS_AUDS are set.

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