feat(command-center): add support for standalone terminal cells

[MattPua]

Problem

Command center only supported adding tasks. No way to open a terminal independent of an agent run.

Changes

CleanShot.2026-07-03.at.11.25.54.mp4

• Adds a Terminal option to the cell selector (alongside New task / Brainrot)
• New __terminal__:<id> cell sentinel in grid.ts, mirroring the brainrot pattern
• TerminalCell component — opens a shell in the most recently accessed folder ($HOME fallback)
• Renamed removeTask store action → clearCell (it clears any cell type, not just tasks)
• Cleanup on remove, drag-overwrite, layout shrink, and clear-all so ptys don't leak
• New destroyShellTerminal helper: kills xterm instance + server-side pty + persisted state

How did you test this?

• pnpm --filter @posthog/core typecheck and pnpm --filter @posthog/ui typecheck — clean
• Unit tests: packages/core command-center suite (46 tests) and packages/ui store suite (1211 tests) — all pass

Automatic notifications

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

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit ada2f9d.}

[greptile-apps]

Comments Outside Diff (1)

1. packages/core/src/command-center/cells.test.ts, line 14-24 (link)

   The brainrot test case was not updated to assert terminalId: null, even though the empty-cell case was. Because EMPTY_CELL_DATA now spreads terminalId: null for all cell types, a brainrot cell built by buildCommandCenterCells will always carry that field. Leaving it out of the expected object lets a future regression (e.g. accidentally propagating a terminal ID into a brainrot cell) slip through undetected.

   Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

_{Reviews (1): Last reviewed commit: "feat(command-center): add support for st..." | Re-trigger Greptile}