feat: add native decision cards

[camelludo]

Summary

• add typed, signed DecisionCard and DecisionResponse events with Markdown fallbacks
• render polished theme-adaptive cards directly in the Buzz timeline
• record four human intents as durable thread receipts: Approve, Redraft, Escalate, Reject
• bind the response to the exact rendered payload hash and fail closed on tampering or prior-response lookup failure
• keep this proof explicitly shadow-only at the operating layer: no external delivery or business mutation

Event contract

• 40009: decision card
• 40010: signed decision response
• h scopes both events to a channel
• e keeps the response in the originating thread
• payload_hash binds the human response to the structured payload
• ordinary Markdown remains readable in older clients

Verification

• cargo test -p buzz-core -p buzz-sdk
• relay scope tests for channel-scoped content and per-kind allowlists
• desktop parser/timeline tests (34 passed)
• desktop TypeScript and targeted Biome checks
• compiled Desktop Playwright proof in light and dark themes (2 passed)
• isolated non-production relay + fresh Postgres round-trip: signed card and response accepted and queried back durably

Boundary

This PR adds a native collaboration/intent surface. It does not call Stomaton, send externally, or treat a Buzz decision as delivery proof.