feat(channels): ship production thread and feed UX

[k11kirky]

Problem

Why: The two open channel-thread experiments overlap, but include conflicting and demo-only behavior. This consolidates the production-ready interaction model while keeping the channel UI aligned with current backend contracts.

Supersedes #3387 and #3287.

Changes

• Preserve docked threads independently per channel and make task cards accessible links with standard new-tab behavior.
• Render prompts, human replies, and agent output chronologically, with live status anchored at the bottom and safe handling for runless local tasks.
• Reconcile stale feed state with settled sessions and recover feed polling after transient failures.
• Retain the shared mention-chip presentation and the local Quill development skill from the source work.
• Keep the context-generation workflow already shipped on main through #3370; exclude only the fake feed composer, placeholder Inbox tab, and other demo-only UI.

How did you test this?

• pnpm build
• pnpm typecheck
• pnpm lint
• node scripts/check-host-boundaries.mjs
• pnpm --filter @posthog/core test
• pnpm --filter @posthog/ui test
• Focused timeline, thread-store, task-navigation, feed-polling, and mention-rendering tests

Automatic notifications

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

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Created with PostHog Code

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit b6faccc.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(canvas): align thread mention render..." | Re-trigger Greptile}

[k11kirky]

/trunk merge