fix(memory): Fix event listener leaks in TUI and Slack bot

[sauerdaniel]

Summary

Fix memory leaks from event listeners not being properly cleaned up in TUI components and the Slack bot.

Fixes #9155

Problems

1. TUI Sync Context (sync.tsx)

The sdk.event.listen() call sets up event listeners but the returned unsubscribe function is never called.

2. Tooltip Component (tooltip.tsx)

Mouse event listeners are added to elements but never removed on component unmount.

3. Slack Bot (slack/index.ts)

• Sessions Map grows without bounds
• No cleanup of old/stale sessions
• No graceful shutdown handling

Solution

TUI Sync Context

Store and call unsubscribe function in onCleanup:

const unsubscribe = sdk.event.listen((e) => { ... })
onCleanup(unsubscribe)

Tooltip Component

Add proper cleanup in onCleanup handler.

Slack Bot

• Add lastUsed timestamp to sessions
• Add periodic cleanup of sessions older than 1 hour
• Add MAX_SESSIONS limit (100)
• Add graceful shutdown handlers for SIGINT/SIGTERM

Changes

• packages/opencode/src/cli/cmd/tui/context/sync.tsx - Store and call unsubscribe
• packages/ui/src/components/tooltip.tsx - Add event listener cleanup
• packages/slack/src/index.ts - Add session management and shutdown handlers

Testing

• [x] TypeScript compilation passes (bun turbo typecheck)
• [x] Unit tests pass (725 tests, 0 failures)

Note: Manual TUI and Slack bot testing was not performed. Memory leak verification requires runtime memory profiling.