test(coverage): remove core exclusions and enforce gates

[OneStepAt4time]

Summary

• remove src/server.ts, src/session.ts, and src/tmux.ts from Vitest coverage exclusions
• add server-core-coverage.test.ts integration coverage test that boots real server wiring (with controlled tmux/pipeline boundaries) and exercises high-value server/session/tmux API paths
• keep existing global thresholds unchanged (lines/statements/functions 70, �ranches 60) while making them pass with core modules included

Validation

• npm run lint
• npm run build
• npx tsc --noEmit
• npm test -- --coverage

Closes #1616