feat: support offline mode with env var version pinning and cache fallback

[fauzxan]

Summary

Fixes #15 — the CLI binary currently always contacts registry.npmjs.org to resolve the latest @holistics/cli-core version, even when the package is already cached locally. This makes the CLI unusable in network-restricted environments.

This PR adds two mechanisms to ensureModule in src/downloader.ts:

• HOLISTICS_CLI_CORE_VERSION env var — when set, the npm registry call is skipped entirely and the specified version is loaded directly from cache
• Offline fallback — when the env var is not set and getLatestVersion() fails due to a network error, the CLI scans ~/.cache/holistics/ for any previously downloaded version (picking the highest) instead of crashing

Changes

src/downloader.ts:

• Added findCachedVersion() — scans the cache directory for existing {pkg}@{version} entries and returns the highest version found
• Modified ensureModule() — checks HOLISTICS_CLI_CORE_VERSION env var first, then attempts npm with fallback to cached version on network failure
• Added readdir import from fs/promises and getCacheDir import from ./cache

Usage

# Pin version explicitly (zero network calls)
HOLISTICS_CLI_CORE_VERSION=0.6.13 holistics dbt upload --file-path manifest.json --data-source my_ds

# Or just work offline if cache exists from a previous run
holistics --version  # falls back to cached version when npm is unreachable

Use case: Pre-baked Docker images

# At build time (network available): download cli-core into cache
RUN holistics --version  # populates ~/.cache/holistics/@holistics/cli-core@x.y.z/

# At runtime (no network): pin the version
ENV HOLISTICS_CLI_CORE_VERSION=0.6.13
RUN holistics dbt upload ...  # works without npm

Test plan

• Verify holistics --version works normally (no env var, network available)
• Verify HOLISTICS_CLI_CORE_VERSION=0.6.13 holistics --version works with pre-populated cache and no network
• Verify offline fallback: run once to populate cache, then disconnect network and run again without env var
• Verify error is still thrown when offline with no cache and no env var

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