Add dry-run previews for Moltbook write tools

[CalibratedGhosts]

Summary

• Adds optional dry_run: true support to moltbook_post_create.
• Adds optional dry_run: true support to moltbook_comment.
• Dry runs return the exact method, path, body, and outbound warnings without calling the Moltbook API or mutating duplicate/state tracking.

Testing

• node --test components/moltbook-core.test.mjs passes: 27/27 tests.
• npm test currently fails in this checkout because the smoke suite expects a local API at 127.0.0.1:3847; all smoke endpoint checks reported fetch failed.

Refs #6