Implement offline build mode

[heartwilltell]

Phase 6: Polish — RFC #218

When all dependencies are cached (or vendored), builds should work without network access. Implement detection of missing cached modules with clear error messages suggesting run mod download.

Acceptance Criteria

• If all modules are cached, no network requests are made during build
• If a module is missing from cache, error: module 'github.com/user/repo@v1.2.3' not in cache; run 'run mod download'
• Support RUNFLAGS=-vendor to force vendor-only resolution
• Document offline workflow in docs

Files

• Modified: src/modcache.zig, src/driver.zig

Dependencies

• Implement run mod download and run mod verify commands #308 (run mod download/verify)
• Implement multi-file compilation for external packages #314 (multi-file compilation)

Part of the Package Manager RFC.