🏗️ refactor(runtime): adopt Effect v4 for the next v5 alpha

[smashah]

What changed

• Adopt Effect v4 as the v5 capability kernel, with runtime-specific browser, Node, Bun, and edge adapters.
• Add scoped browser/session resource management, capability-enforced chat sandboxes, supervised plugin services, durable webhook delivery, and parallel startup work.
• Replace PQueue paths with measured Effect-backed concurrency primitives and add observability, benchmarks, and contract evidence.
• Move source imports to extensionless TypeScript paths and upgrade the workspace toolchain, including TypeScript 7 and pnpm 11.15.1.
• Make clean-checkout validation deterministic and add a Changesets entry for the next v5 alpha.

Why

This implements the Effect v4 architecture explored in #3385 while the v5 API is still intentionally negotiable. It gives each supported runtime an explicit capability boundary, improves browser-session cleanup and memory control, and lets independent startup work such as patch and license preloads run concurrently.

Release impact

• The repository stays in the existing alpha prerelease mode.
• The new runtime adapter packages join the fixed v5 release group.
• After merge, the normal master to release promotion can publish the next alpha after v5.0.0-alpha.8.

Validation

• pnpm install --frozen-lockfile on Node 22.22.2 / pnpm 11.15.1
• pnpm test — 64/64 Turbo tasks passed, including docs build and prerender
• pnpm typecheck — 16/16 tasks passed
• pnpm lint
• pnpm check:effect-contracts
• pnpm changeset status
• git diff --check

The startup benchmark measured about 2.07x speedup for one session and 6.25x for three concurrent sessions.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch agent/effect-v4-alpha

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