feat(canvas): keep task threads human-only

[raquelmsmith]

Problem

Task threads had become noisy: the agent's live conversation (its turns, the prompts sent to it, and a running status line) was mirrored into the thread panel alongside human comments. This broke the thread as a focused space for human↔human discussion. Raised in a Slack thread by the canvas/channels users.

Changes

• Task threads are now human-only. The thread panel no longer mirrors the agent's live turns, the prompt echoes, or the agent status line.
• Removed the live-session wiring in the panel (useSessionViewState/useSessionConnection/useConversationItems) that was subscribing to the agent session purely to feed the thread — this also stops pinning an agent process alive just for the panel.
• Simplified buildThreadTimeline to a single human-message row kind and dropped the now-unused agent-turn/status helpers.
• The human @agent forward action (task author sending a thread message to the agent) is unchanged, as are backend-authored announcement rows.

How did you test this?

• pnpm --filter @posthog/core --filter @posthog/ui typecheck — clean.
• pnpm --filter @posthog/core exec vitest run src/canvas/threadTimeline.test.ts — pass.
• pnpm --filter @posthog/ui exec vitest run src/features/canvas/components/ThreadPanel.test.tsx — pass.
• biome check on the changed files — clean.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog from a Slack thread

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 3d51286.}