fix(createSandbox): report token usage on reused sandboxes

[masone]

The bug

When you reuse a sandbox across multiple runs:

const sandbox = await createSandbox({ branch, sandbox: docker(...) })
const result = await sandbox.run({ agent, prompt })
result.iterations[0].sessionFilePath // ❌ always undefined
// → no session file captured, so no token usage is reported

The one-shot run() reports token usage fine. Only the reused
createSandbox + sandbox.run() path comes back empty.

Why

After each run, the orchestrator copies the agent's session file out of the
sandbox to the host and reads token usage from it. That step only happens when
it has a handle to the sandbox's filesystem (bindMountHandle).

run() passes that handle through. createSandbox builds its own internal
factory to reuse the container across runs — and that factory was passing
everything except the handle. So the copy-out step was silently skipped every
time, and with no session file there was nothing to read usage from.

The fix

Forward the bind-mount handle through the reuse factory, the same way run()
already does. The handle is narrowed to the bind-mount provider case (so an
isolated/no-sandbox handle is never mistaken for a bind-mount one). With the
handle present, the existing capture code runs and sessionFilePath / token
usage are populated on reused sandboxes too.

Changes

• src/createSandbox.ts — carry the bind-mount handle into the handle context
  and forward it into the orchestrator's per-run context, narrowed to
  bind-mount providers.
• src/createSandbox.test.ts — two regression tests (see below).
• .changeset/ — patch bump.

Tests

Added two tests at the createSandbox level:

• reused bind-mount sandbox captures the session + reports usage —
  sessionFilePath and usage are populated. Verified to fail on the
  pre-fix factory (expected undefined to be defined).
• a non-bind-mount (isolated) provider does not capture — a sessionId is
  still extracted, but the handle isn't forwarded, so capture is skipped. This
  pins the narrowing.

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