feat(install-vm): install gateway + vm driver, add --driver-dir resolution

[drew]

Summary

Reworks install-vm.sh to bootstrap a working VM-driver gateway end-to-end, and teaches the gateway how to find helper binaries in a dedicated driver directory so they don't have to sit on PATH. Running the script now leaves you with everything needed to start a gateway that auto-detects the VM compute driver.

Related Issue

N/A — maintenance / install UX.

Changes

install-vm.sh

• Downloads two binaries instead of one:
  • openshell-gateway from the rolling dev release → OPENSHELL_INSTALL_DIR (default ~/.local/bin).
  • openshell-driver-vm from the rolling vm-dev release → OPENSHELL_DRIVER_DIR (default ~/.local/libexec/openshell), which matches the gateway's new default.
• Verifies checksums against each release's checksum file.
• Codesigns only openshell-driver-vm on macOS (it's the process that runs libkrun; the gateway doesn't need the entitlement).
• Drops the old openshell-vm / OPENSHELL_VM_INSTALL_DIR naming.
• Prints the exact openshell-gateway --drivers vm … command to start the gateway at the end; omits --driver-dir when the default is in use so the one-liner stays short.

crates/openshell-server

• New --driver-dir / OPENSHELL_DRIVER_DIR flag on openshell-gateway, default $HOME/.local/libexec/openshell.
• VmComputeConfig gains a driver_dir: Option<PathBuf> with a default_driver_dir() helper.
• resolve_compute_driver_bin now searches in order:
  1. Explicit --vm-compute-driver-bin override.
  2. {driver_dir}/openshell-driver-vm.
  3. Sibling of the gateway executable (kept so local cargo run still works).
• Error message enumerates every searched path and the relevant flags/env vars.
• Four new unit tests cover each resolution branch and the error case.

Testing

• mise run pre-commit passes
• Unit tests added/updated — 4 new tests in compute::vm::tests
• E2E tests added/updated (not applicable — no runtime behavior change beyond the new flag)

Manual verification:

• cargo test -p openshell-server --lib compute::vm::tests → 6/6 pass.
• OPENSHELL_INSTALL_DIR=/tmp/x OPENSHELL_DRIVER_DIR=/tmp/y sh install-vm.sh → both binaries downloaded, checksummed, codesigned on macOS, installed to the requested dirs, and the correct start command is printed.
• ./target/debug/openshell-gateway --help shows the new --driver-dir flag.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (no architecture doc covers the install script or driver resolution today; happy to add one if preferred)

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