feat(channels): add scoped channel foundations

[puemos]

Problem

Channel spaces need project-safe identity, item loading, and task creation semantics before any visible layout can be activated.

Changes

• Add current-channel state, validation, and project/logout cleanup.
• Add merged channel task/canvas models with ownership, filtering, pinning, and archiving.
• Add channel-aware task creation and prefill handling.
• Define the feature gates without connecting them to visible layout code.

This is PR 2 of the stack, based on #3812. With either flag state, users still see the existing layout.

How did you test this?

• pnpm --filter @posthog/core test -- src/canvas/channelItems.test.ts src/canvas/runStatus.test.ts
• Five focused UI suites covering channel identity, task routing, prefills, and refreshed tasks (23 tests)
• pnpm typecheck in packages/ui

Automatic notifications

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

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Created with PostHog Code

[github-actions]

React Doctor found 1 issue in 1 file · 1 warning.

1 warning

src/features/task-detail/components/TaskInput.tsx

• ⚠️ src/features/task-detail/components/TaskInput.tsx:286 Event logic handled in an effect no-event-handler

_{Reviewed by React Doctor for commit f60bbca.}

[puemos]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(channels): add the structured Activity view #3815
• feat(channels): add the flag-gated spaces layout #3814
• feat(channels): add scoped channel foundations #3813 👈 (View in Graphite)
• refactor(canvas): share thread and PR artifact logic #3812
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.