feat(sandbox): add smolvm sandbox mode

[chriswritescode-dev]

Summary

Adds a smolvm sandbox backend alongside the existing sbx backend, selected via sandbox.mode. The smolvm CLI embeds libkrun and drives the local hypervisor directly, with no daemon.

Behavior

• src/sandbox/smolvm.ts (new): smolvm runtime — machine create from a docker save tar image store at <dataDir>/smolvm-images/, machine exec shell routing, per-machine --allow-host network flags from sandbox.network.allow, env-file sourcing in-guest, and transparent restart-then-retry when a machine was stopped out-of-band.
• src/sandbox/runtime-factory.ts (new): createSandboxRuntime is the single mode-dispatch point; resolveSandboxMode resolves sandbox.mode ('sbx' default, 'smolvm'), falling back to 'sbx' for unknown/legacy values.
• src/sandbox/sbx.ts and src/sandbox/shell-shim.ts: backend-neutral rework — describeUnavailable and templateLoadHint moved onto the runtime; the generated shell shim routes through smolvm machine exec when mode is smolvm and fails closed exactly like the sbx shim.
• src/sandbox/path.ts, manager.ts, template.ts, types.ts, src/index.ts, src/tui.tsx: mode-aware wiring — resolveSandboxEnvDir, network allowlist passthrough, per-mode template build/store commands, and SandboxMode typing.
• Network semantics differ by design: smolvm machines have no global proxy, so an empty sandbox.network.allow means unrestricted egress (the opposite of sbx's deny-by-default proxy); host loopback stays unreachable.
• Docs and tests updated: smolvm-runtime.test.ts, runtime-factory.test.ts, sbx-runtime.test.ts, shell-shim.test.ts, manager-network-allow.test.ts, plus helpers.

Notes

• No open pr-review ledger findings touch this change set.
• This branch predates the merged keep-alive rework (PR feat(sandbox): keep active sandboxes warm with sentinel keep-alive exec #91); its first two commits are a parallel implementation of that same sentinel keep-alive work, so the PR diff shows overlapping keep-alive content and docs/sandbox.md conflicts against main. The new work here is the smolvm backend.

Validation

pnpm typecheck and pnpm lint clean; sandbox runtime tests pass.

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