cross-cutting: prefill / programmatic compose API (capability-flag style)

[productdevbook]

Some providers support message prefill (Beacon, Tidio, Jivo, Drift interactions), most don't (Chatwoot, LiveChat native no). Design as capability flag.

Evidence

• `Trigger Chat via SDK with pre-defined prompt chatwoot/chatwoot#2998` — "Trigger Chat via SDK with pre-defined prompt" Trigger Chat via SDK with pre-defined prompt chatwoot/chatwoot#2998
• Beacon prefill: `Beacon('prefill', {...})`
• Tidio: `tidioChatApi.messageFromVisitor(text)`
• Drift: `drift.api.startInteraction({interactionId})`

Tasks

• `prefill({ subject?: string; text?: string; fields?: Record<string, unknown> })`
• Adapter capability type: `supports: { prefill: boolean, track: boolean, identityHmac: boolean, jwt: boolean, unreadCount: "native" | "poll" | "none", locale: boolean, theme: "config" | "runtime" | "none" }`
• Unsupported calls throw `UnsupportedOperationError` with capability matrix link

[productdevbook]

Implemented in this commit