[CHORE] Install tmux in Containerfile for worktree session persistence

[gerchowl]

Chore Type

Configuration change

Description

The justfile.worktree recipes rely on tmux to run autonomous cursor-agent sessions in detached tmux sessions (tmux new-session -d -s "wt-<issue>"). However, tmux is not included in the Containerfile's apt-get install list (line 53-62), so just worktree-start fails at the prerequisite check.

This is especially important for the remote devcontainer use case: worktree agents must survive Cursor session disconnects, and tmux is the persistence mechanism that makes this possible. Without tmux, the autonomous TDD workflow (RED-GREEN-REFACTOR with per-phase commits as defined in .cursor/rules/tdd.mdc) cannot run to completion — a disconnected session kills the agent mid-cycle, leaving partial test/implementation commits and breaking TDD compliance in the git history.

Acceptance Criteria

• tmux is installed via apt-get in the Containerfile alongside existing system dependencies
• Container image builds successfully with tmux included
• just worktree-start no longer fails the tmux prerequisite check inside the container
• Autonomous worktree agents can complete full TDD cycles (test commit → implementation commit → refactor commit) after Cursor session disconnect
• Unit test: test_tmux_installed added to TestSystemTools in tests/test_image.py — verifies tmux apt package is installed (follows existing pattern: host.package("tmux").is_installed)
• Unit test: test_tmux_version added to TestSystemTools — verifies tmux --version returns successfully and matches expected major version
• tmux version added to EXPECTED_VERSIONS dict in tests/test_image.py
• Integration/smoke test: test_tmux_detached_session_survives added to tests/test_image.py — creates a detached tmux session, verifies it appears in tmux list-sessions, and confirms the session's background process is running (validates the core persistence mechanism worktree agents rely on)

Implementation Notes

• Add tmux \ to the existing apt-get install block in the Containerfile (line 53-62). No other changes should be needed — the justfile already has the tmux check and usage in place.
• Unit tests follow the exact pattern of test_nano_installed / test_git_version etc. in TestSystemTools.
• The smoke test should create a detached session (tmux new-session -d -s test-session "sleep 60"), verify it with tmux list-sessions, then clean up (tmux kill-session -s test-session). This validates the same mechanism justfile.worktree uses without needing the full worktree infrastructure.

Related Issues

None

Priority

Medium

Changelog Category

Added

Additional Context

The justfile.worktree already validates tmux is present and errors with a helpful message if missing. This issue just closes the gap so the container is ready out of the box for autonomous TDD workflows.