refactor(compute): decouple gateway driver composition

[drew]

Summary

Move first-party compute-driver composition and VM process ownership into openshell-gateway, leaving openshell-server backend-independent. Packaging, build references, driver-free build tooling, and final boundary cleanup land together so this architectural move is complete and buildable on its own.

Related Issue

Part of #2417.

Stacked on #2822.

Changes

• add openshell-gateway as the standard binary and first-party composition crate
• gate compiled first-party registrations behind in-tree-compute-drivers
• move VM subprocess launch and first-party registration out of openshell-server
• remove compute-driver implementation dependencies and backend construction from openshell-server
• remove compute-backend construction details from openshell-core
• update Cargo, Bazel, release, packaging, deployment, and development references for the gateway crate
• remove stale backend-specific server comments
• keep the driver-free gateway path portable with bundled Z3 and paired CI CMake tooling
• simplify the final registry/factory boundary

The resulting tree is byte-for-byte identical to the pre-split #2786 head.

Testing

• mise run pre-commit
• mise run e2e:gateway:no-compute-drivers
• final tree comparison against the pre-split refactor(compute): register compiled drivers #2786 head
• full external-driver E2E matrix is covered by branch CI

Checklist

• Changes are scoped to accepted issue refactor(compute): define driver shutdown and restart lifecycle contract #2417
• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture documentation is updated
• Public-facing documentation remains unchanged
• PR remains draft

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[drew]

/ok to test

[drew]

/ok to test

[drew]

/ok to test

[drew]

/ok to test

[elezar]

@drew This branch update is rebase-only: PR #2823 was rebased onto the restored PR #2822 head. No intentional feature or scope changes were added to this PR; the conflict resolution preserves the parent stack behavior while retaining the existing gateway-composition and telemetry commits.

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[drew]

/ok to test

[drew]

/ok to test