feat: add /new to clear chat

[igennova]

Problem

Users (especially in Command Center) want a slash command to clear the current chat and start fresh — the same muscle memory as /new / /clear in agent CLIs.

Part of PostHog/posthog#76323

Changes

• Add a built-in /new code command in the message editor autocomplete
• Add SessionService.startFreshSession: tear down the current run and create a new task run so the transcript / logs start blank
• Wire /new through useSessionCallbacks via an injected onNewSession callback (keeps commands free of session-service locators)
• Local chats only; cloud shows a toast if /new isn’t available

Screenshot

How did you test this?

• Unit tests: commands.test.ts, getSuggestions.test.ts, sessionServiceRetryConfig.test.ts (startFreshSession + resetSession unchanged)
• Manual: type /new in a local session and confirm the chat clears to a blank transcript

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[github-actions]

This PR has had no activity for 7 days and has been marked stale. We are moving to the monorepo and tightening PR staleness in preparation, so it will be closed in 7 days if no further activity occurs.

[charlesvien]

Closing stale PRs ahead of Friday's monorepo migration. If this is a mistake, rebase and reopen for review.