logger: add optional debug() as a diagnostic-only logging seam

[TheGreatAxios]

Summary

• Adds an optional debug(message: string): void to tag-slack's Logger type, alongside the existing warn. defaultLogger routes it to console.debug.
• warn is for fail-soft conditions a host should see by default; debug is for diagnostic-only lines a host can opt into without adopting a full log-level system. Optional so existing host loggers (which only implement warn) keep compiling and working unchanged — an absent debug means debug lines are silently dropped, never an error.
• No call site in this PR. Stacked on Reconcile TagThread.post(): convertMarkdown opt-out + Block Kit blocks/unfurl suppression #6 (not because it depends on that code, but to keep CI green-before-stacking per review order); the first caller (a slash-command diagnostic) lands with the slash-commands slice later in this series.

Test plan

• CI: bun run typecheck && bun run test
• Type-only/additive change to an existing seam with no existing dedicated test file; covered indirectly by every existing wire.test.ts/index.test.ts case that still passes with the widened Logger type.