fix(cli): skip auto system sessions on continue

[netsky-prod]

Summary

• Add shared continue-session selection that skips known auto system sessions (Auto Dream, Auto Distill).
• Use the selector in both TUI mimo -c and headless mimo run -c.
• Keep skipping child sessions, so checkpoint/subagent sessions remain excluded from continue candidates.
• Add regression coverage for auto-system and child-session filtering.

Fixes #1484.

Verification

• RED before fix: bun test test/cli/continue-session.test.ts --timeout 30000 failed because the selector module did not exist.
• bun test test/cli/continue-session.test.ts test/cli/tui/route-agent-id.test.ts test/cli/tui/sync-bucket.test.ts --timeout 30000 - 8 pass, 0 fail.
• bun typecheck
• bunx oxlint packages/opencode/src/cli/continue-session.ts packages/opencode/src/cli/cmd/run.ts packages/opencode/src/cli/cmd/tui/app.tsx packages/opencode/test/cli/continue-session.test.ts - 0 errors, existing warnings only.
• git diff --check
• Pre-push bun turbo typecheck - 12 successful, 12 total.