[Feat]: Integrate artifacts into task `history`

[matoushavlena]

Is your feature request related to a problem? Please describe.

Artifacts (files, images, charts, code outputs) are stored separately from history, making it impossible to fully reconstruct or replay conversations where artifacts are interleaved with messages.
In multi-agent workflows, artifacts are often produced alongside related commentary (e.g., chart → analysis → code → commentary). When artifacts are detached from the message flow, later agents can’t access them in context, reducing the ability to reference, reason over, or transform them.

Describe the solution you'd like

Allow artifacts to appear inline in history alongside messages, using a discriminator to clearly identify the type.
This would preserve chronological order and make parsing easier for clients.
Proposed schema change:

type HistoryItem = { kind: 'message' } & Message | { kind: 'artifact' } & Artifact;

history?: HistoryItem[];

Describe alternatives you've considered

Keeping artifacts separate but adding explicit cross-links to their related messages

Additional context

Issue #301 also addresses preserving chronological task data, but focuses on status updates rather than artifacts.

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