feat(messaging): surface Plan/Review artifacts via Markdown attachment + inline preview

[huntharo]

Summary

Refs #193.

This adds the generic Plan/Review artifact delivery path for messaging providers. Artifacts now render through capability-driven intent generation instead of provider branches:

• short artifacts send inline only
• long artifacts send a bounded inline preview plus a Markdown attachment when outboundAttachments.supportsFileUpload and maxUploadBytes allow it
• text-only providers and oversized artifacts get a longer inline fallback
• failed attachment delivery retries once as inline fallback

Changes

• Adds messaging-artifact-renderer with shared Plan/Review artifact formatting and attachment selection.
• Caches turn/plan/updated by turn in MessagingController and delivers the artifact on terminal turn completion.
• Adds generic review artifact delivery for completed review items using the same renderer.
• Replaces the status-card Plan mode: unavailable stub with capability-based plan delivery status.
• Marks Telegram as supporting outbound file upload, matching its existing sendDocument delivery path.
• Updates messaging capability docs now that outboundAttachments has a producer consumer.

Verification

• pnpm test apps/desktop/src/main/__tests__/messaging-artifact-renderer.test.ts apps/desktop/src/main/__tests__/messaging-controller.test.ts apps/desktop/src/main/__tests__/messaging-status-card.test.ts
• pnpm --filter @pwragent/desktop typecheck
• pnpm --filter @pwragent/messaging-interface typecheck
• pnpm --filter @pwragent/messaging-provider-telegram typecheck
• pnpm lint:boundaries

Notes

Manual real-bot validation for Discord and Telegram attachment delivery is still worth doing before considering #193 fully closed.