Add --dry-run to the manifest-mutating scripts

[collinsadi]

scripts/update-artifact-manifest.ts and scripts/update-manifest-wasm-hashes.ts write files directly with no preview, unlike deploy-contracts.ts, which already supports --dry-run. We should add a --dry-run flag that prints the computed hash diffs without writing.

Acceptance criteria

• Both scripts accept --dry-run, print a diff, and write nothing.
• It is documented in the script help and the README command reference.