fix(tui): scroll to bottom when returning from subagent

[netsky-prod]

Summary

• key the TUI conversation scroll effect by both sessionID and active agentID
• make main/subagent switches within the same session call toBottom() when returning to the rendered conversation
• add a regression helper test for the scroll key behavior

Fixes #1433

Verification

• RED: bun test test/cli/tui/session-scroll.test.ts --timeout 30000 failed before the helper existed
• cd packages/opencode && bun test test/cli/tui/session-scroll.test.ts test/cli/tui/route-agent-id.test.ts test/cli/tui/sync-bucket.test.ts --timeout 30000 (8 pass)
• cd packages/opencode && bun typecheck
• bunx oxlint packages/opencode/src/cli/cmd/tui/routes/session/index.tsx packages/opencode/src/cli/cmd/tui/routes/session/scroll.ts packages/opencode/test/cli/tui/session-scroll.test.ts (0 errors; existing warnings in index.tsx)
• git diff --check
• pre-push: bun turbo typecheck (12/12 successful)