Support custom E2B endpoints for managed sandboxes

[sam2tom]

Summary

• Add paired e2b.api_url and e2b.domain to the managed deployment configuration, with official E2B defaults, strict HTTPS/DNS validation and a same-domain control/data-plane relationship.
• Persist the selectors, pass them explicitly to the pinned E2B SDK for template validation, lifecycle and metrics, and bind allocation receipts to the endpoint identity. Reject a returned sandbox domain before any envd command or file operation.
• Expose the configuration in the Core client, Web setup and installer; update /core/v1 contracts, generated OpenAPI, documentation and regression tests.

Tracks #179; its Core-managed acceptance remains open.

Validation

• make sqlc-generate, make openapi, make check-sqlc, make check-go: passed.
• TMPDIR=/private/tmp make check-agents-api: passed; database integration tests skip because OAC_TEST_DATABASE_URL is unset.
• Pinned E2B SDK 2.51.0 helper suite: 104 passed. Installer E2B setup tests: 3 passed.
• pnpm typecheck, agents-client suite (650), Web suite (386), docs build and docs verification: passed.
• Independent blind review completed; both DNS validation findings were corrected and retested.

Remaining acceptance

• Run a Core-managed Session and daemon end to end against SandBase Sandbox, including template lookup, allocation, bootstrap, command/file I/O, observation, renewal, cleanup and interrupted-create reconciliation. Prior direct SDK qualification does not prove this Core path.
• Run the new Store persistence/switch test against a dedicated PostgreSQL. A temporary PostgreSQL container could not start because local Docker containerd storage returned no space left on device; the test currently skips without OAC_TEST_DATABASE_URL.
• Full make check stops in unrelated Linux-only installer tests on this macOS host (Linux host preflight and simulated UID). The Core and Web checks above were run separately.

No deployment or merge is included.

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[sam2tom]

Superseded by #196, which now contains this custom E2B endpoint work, the SDK Create fix, and #204 provider setup/template discovery. #196 has been updated against current main and is the single integration PR.