feat: support repeated -e/--env-file for merging multiple env files

[BoldBlackBot]

Summary

-e/--env-file was previously a single-value option. Passing it twice silently dropped the first file (minimist last-value-wins). This PR makes it repeatable, mirroring the existing --volumes pattern.

Motivation

Real use case: certain harness invocations need a mix of env files (e.g. a base secrets file + a project-specific overrides file). Without this, you'd have to create a merged env file for every permutation.

Changes

• Normalize argv["env-file"] into string[] (same pattern as --volumes), so both single and repeated values work
• Emit one docker --env-file <path> flag per input file via flatMap
• Validate each file exists (loop instead of single check)
• Update AgentOptions.envFilePath → envFilePaths: string[] (only PiAdapter reads it, for the local-mode provider decision)
• Update USAGE text: ; may be repeated (matching -v, --volumes)
• Cloud-mode detection (HARNESS_CLOUD_MODE) now checks envFilePaths.length > 0

Backward compatibility

Single -e file.env works identically — the normalize step wraps it in an array.

Test plan

• pnpm build — clean compile (tsc --noEmit)
• pnpm lint:ts — biome clean
• pnpm test:e2e — 117/117 pass, including 3 new tests:
  • repeated -e passes all env files as separate --env-file flags
  • both files are present and resolved to abs paths
  • --help documents -e may be repeated
• All pre-existing env-file and cloud-mode tests still pass unchanged

[BoldBlackBot]

Build failure is a transient NodeSource outage — not from this PR

The build jobs (amd64 + arm64) failed with:

curl: (22) The requested URL returned error: 403
curl -fsSL https://deb.nodesource.com/gpgkey/nodesource-repo.gpg.key

This is the base Dockerfile fetching NodeSource's GPG key during image build. NodeSource's endpoint is currently returning 403 (confirmed live just now). The same build job passed on chore/bump-deps ~6.5h ago with the identical Dockerfile.

This PR is TypeScript-only (no Dockerfile changes):

src/harness.ts             | 38 +++++++++++++++++++++++---------------
tests/e2e/helpers.mjs      |  4 ++++
tests/e2e/parsing.test.mjs | 25 +++++++++++++++++++++++++

lint ✅ and e2e ✅ both pass. The build will go green once NodeSource recovers. Happy to rerun it then.