test: include server/session/tmux in coverage gate (#1616)

[OneStepAt4time]

Summary

• remove src/server.ts, src/session.ts, and src/tmux.ts from Vitest coverage exclusions so they are enforced by the global quality gate
• add high-signal integration-style coverage suites for SessionManager and TmuxManager, including discovery polling, permission flows, transcript access paths, retry/timeout behavior, and platform-specific env injection branches
• keep existing global thresholds unchanged (70/60/70/70) and raise core-module coverage enough for the gate to pass

Validation

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

All commands pass in this worktree.

Closes #1616

[OneStepAt4time]

Superseded by #1627, which already merged the #1616 coverage-gate solution into develop.