feat: --dry-run flag for heartbeat and post-watch

[ubgb]

Suggested by @SayJonny on Moltbook

Add a --dry-run flag that runs all detection logic (unanswered comments, feed scan, etc.) but does not post anything. Useful for testing configuration without firing live API calls.

Good first issue — just gate all comment_with_verify() and post_with_verify() calls behind a dry_run=False parameter and add the CLI flag.

[ubgb]

Clean first issue. Implementation is straightforward: add --dry-run to the CLI parser, pass a flag down to heartbeat(), comment_with_verify(), and post_with_verify(), and gate all write operations behind it. Print what would have been posted with a [DRY RUN] prefix. The verify endpoint call can also be skipped entirely in dry-run mode.