Guide E2B provider setup with official v2 template discovery

[sam2tom]

Summary

• Add SandBase, E2B, and custom E2B-compatible provider choices to the sandbox setup wizard. Presets fill the API origin and data-plane domain; operators can edit them.
• Discover account-visible templates with the official E2B Python SDK 2.51.0 GET /v2/templates operation, then list ready exact builds for one selected template. Discovery uses a transient credential through Core-key-only /core/v1 routes; deployment save still validates the exact build.
• Update the Core client, OpenAPI, generated reference, API index, and operator docs. Discovery never allocates compute or stores the credential.

Dependency and external blocker

• Base this PR on codex/e2b-create-metadata-fix (PR Integrate E2B-compatible providers and template discovery #196), which contains the E2B create qualification fix and the custom endpoint support it depends on. Retarget to main after that PR merges.
• SandBase production currently returns 200 for GET /v2/templates?limit=100, but its array items are missing fields required by the official SDK Template model; SDK parsing stops at KeyError: 'buildID'. The setup wizard reports discovery unavailable until Sandbox returns the official v2 shape. We intentionally do not add a provider-specific response adapter.

Verification

• Pinned E2B SDK Python helper tests: 127 passed, including v2 pagination and build filtering.
• Focused Go API/provider tests passed; full API/server Go packages passed before the final v2-only helper change.
• Agents client tests: 652 passed. Core Web production build and TypeScript checks passed. Core Web suite passed once (386 tests); later reruns hit unrelated enrollment-command test timeouts under local load.
• pnpm --dir apps/docs verify passed. make openapi and both docs generators ran.
• Independent blind review completed with no remaining actionable finding.
• make check was attempted; it stops in the Linux node installer tests on this macOS host (getpwuid/Linux prerequisite failures), before the separate dedicated PostgreSQL prerequisite.

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