Make ConfigManager and cache paths configurable

[mgoldsborough]

Summary

ConfigManager and getCacheDir() / getLocalCacheDir() hardcode their base directory to ~/.mpak/. This prevents downstream consumers (other CLIs, SDKs, embedded usage) from isolating their mpak state to a different directory.

Current behavior

ConfigManager (packages/cli/src/utils/config-manager.ts):

constructor() {
  this.configDir = join(homedir(), ".mpak");
  this.configFile = join(this.configDir, "config.json");
}

getCacheDir (packages/cli/src/commands/packages/run.ts):

export function getCacheDir(packageName: string): string {
  const cacheBase = join(homedir(), ".mpak", "cache");
  ...
}

getLocalCacheDir (same file):

export function getLocalCacheDir(bundlePath: string): string {
  ...
  return join(homedir(), ".mpak", "cache", "_local", hash);
}

All three are hardcoded. No constructor parameter, no env var, no override path.

Proposed changes

1. ConfigManager: accept optional baseDir

constructor(baseDir?: string) {
  this.configDir = baseDir ?? join(homedir(), ".mpak");
  this.configFile = join(this.configDir, "config.json");
  this.ensureConfigDir();
}

No-arg construction retains current behavior (~/.mpak/). Passing a path overrides it.

2. getCacheDir / getLocalCacheDir: accept optional baseDir

export function getCacheDir(packageName: string, baseDir?: string): string {
  const cacheBase = join(baseDir ?? join(homedir(), ".mpak"), "cache");
  const safeName = packageName.replace("@", "").replace("/", "-");
  return join(cacheBase, safeName);
}

export function getLocalCacheDir(bundlePath: string, baseDir?: string): string {
  const absolutePath = resolve(bundlePath);
  const hash = createHash("md5").update(absolutePath).digest("hex").slice(0, 12);
  return join(baseDir ?? join(homedir(), ".mpak"), "cache", "_local", hash);
}

3. Environment variable fallback (optional)

Consider supporting MPAK_HOME as a fallback before the default:

const defaultBase = process.env["MPAK_HOME"] ?? join(homedir(), ".mpak");

This would be consistent with how MPAK_WORKSPACE already works for workspace paths, and follows the pattern of tools like CARGO_HOME, NPM_CONFIG_PREFIX, GOPATH, etc.

4. Export ConfigManager from SDK (optional, separate issue)

Currently ConfigManager is CLI-internal. If other tools need to read/write mpak config programmatically, it should be exported from @nimblebrain/mpak-sdk or a new @nimblebrain/mpak-config package.

Files affected

• packages/cli/src/utils/config-manager.ts — Constructor signature
• packages/cli/src/commands/packages/run.ts — getCacheDir(), getLocalCacheDir()
• packages/cli/src/commands/config.ts — Update ConfigManager() call sites (no change needed if no-arg default preserved)

Backward compatibility

Fully backward compatible. All changes are additive optional parameters with defaults matching current behavior.