fix: preserve manifest formatting when setting packageManager

[Aschenbath]

Writing packageManager currently serializes the entire manifest, removing blank lines and normalizing unrelated values. Update or append only the top-level property so automatic pinning, use, and up preserve the rest of the file byte for byte.

The updater validates the JSON before locating property boundaries. It handles nested values, escaped and duplicate keys, existing non-string values, CRLF/LF, tabs, and manifests without a trailing newline. No dependency is added.

Validation on Node.js 24.20.0:

• Bundle and shims built successfully.
• Full Vitest suite in replay mode with retries disabled: 9 files passed; 192 tests passed and 1 existing expected failure (193 total).
• Repository-wide ESLint with --max-warnings=0, TypeScript, and git diff --check passed.
• 17 updater tests and four CLI regressions cover insertion, replacement, up, and automatic pinning.

The local runner isolates inherited proxy settings from CLI fixtures while retaining proxy access for the live npm-key check. No tests were skipped for this environment adjustment.

Closes #504.

Implementation and automated validation were performed by Codex.

[aduh95]

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