perf(container): remove npm cache from runtime image

[Yevanchen]

What changed

• remove npm's download cache from the existing runtime-package image layer
• keep all installed Claude, Codex, OpenCode, Python, pip, and Driver artifacts unchanged

Why

The cache is build-time residue that is never read at runtime. Keeping it inflates every cold image transfer without adding a runtime capability.

Before / after

Measured on linux/amd64 from Driver main 0db020a, with a byte-identical Driver bundle:

• uncompressed image: 2,360,829,157 B -> 1,788,930,473 B (-24.22%)
• docker save | gzip -n -1: 1,068,600,282 B -> 693,094,196 B (-35.14%)

Verification

• vp fmt --check
• vp run lint
• vp run tc
• vp run test (1,063 passed, 20 live-provider tests skipped)
• vp run build and executable artifact checks
• image smoke: Driver, Claude, Codex, OpenCode, Python, and pip

This PR makes an image-size claim only; it does not claim a causal TTFT improvement.

[Yevanchen]

Clean-context review (delegated): APPROVE. Same-RUN-layer cache clean (real layer shrink, after all install verifications); runtime touches only ~/.npm/_cacache — installed trees and the mosoo-claude-code symlink untouched; no npx/--offline usage repo-wide; claims match staging measurements (pull/disk win, no boot-floor claim). Measured −24.2% uncompressed / −35.1% gzip. CI 3/3 green. Follow-up: superproject pin bump (currently d179ed2) to reach production.