fix(sandbox-driver): honor cwd + env passed to SubprocessSandboxDriver constructor

[drewstone]

Summary

SubprocessSandboxDriver's constructor accepted any-shaped options (no declared interface) and the exec() method only read cwd / env from the per-call HarnessConfig. Calling new SubprocessSandboxDriver({ cwd }) therefore silently did nothing — the subprocess inherited the Node process's cwd instead of the caller's intent.

How this surfaced

End-to-end probing in tangle-network/starter-foundry — the Gen 8 compile-gate thesis is "run pnpm exec tsc --noEmit on the composed scaffold to catch type errors at proposal time." The promoter did:

const driver = new SubprocessSandboxDriver({ cwd: composedOutDir })
// ...
session.ship({ harness: { setupCommand, testCommand, timeoutMs } })

HarnessConfig did not set cwd. The driver read config.cwd → undefined → spawn inherited the Node process cwd (the starter-foundry repo root). Every tsc --noEmit ran against starter-foundry's own (clean) source tree, so the build-gate scored 1.0 on every proposal — including one with a verified TS2345 error I planted as a probe.

starter-foundry shipped a workaround (spread cwd into HarnessConfig at call sites — PR #53), but the silent-drop belongs at this layer: a constructor that accepts { cwd } should honor it.

Changes

• New SubprocessSandboxDriverOptions interface documents the constructor shape.
• Constructor stores { cwd, env } as defaults.
• exec() uses config.cwd ?? this.defaultCwd — per-call wins, constructor is the fallback.
• exec() merges constructor env + per-call env (per-call wins on collision).

Backwards-compatible: callers that only use config.cwd see no change because the fallback is lower-priority.

Tests

+3 in tests/sandbox-harness.test.ts covering:

• constructor cwd is used when HarnessConfig.cwd is unset
• per-call HarnessConfig.cwd overrides constructor cwd
• constructor env merges with per-call env (per-call wins on collision)

319/319 → 322/322. No existing tests changed.

Test plan

• pnpm build — clean
• pnpm test — 322/322
• New test fails without the src change (verified by reverting src temporarily)
• Once merged: bump to 0.8 + update starter-foundry to prefer the cleaner constructor-cwd pattern