feat: zmx session persistence for pixels console

[deevus]

Summary

• pixels console now attaches to a persistent zmx session by default, keeping shells alive across SSH disconnects (network drops, laptop sleep, terminal close). Reconnecting re-attaches to the same session with all processes still running.
• New --session/-s flag (default "console") to name sessions, --no-persist to skip zmx. Graceful fallback when zmx is not installed.
• New pixels sessions <name> command to list active/exited zmx sessions in a container.
• Refactors all SSH functions to use a ConnConfig parameter object, replacing the repeated (host, user, keyPath) triple.

Test plan

• go build compiles
• go test ./... passes (including new TestConsoleArgs cases)
• pixels console <name> attaches to zmx session "console"
• Disconnect and pixels console <name> re-attaches to same session
• pixels console <name> -s build creates/attaches to "build" session
• pixels sessions <name> lists active sessions
• pixels console <name> --no-persist opens plain SSH (no zmx)
• Container without zmx: pixels console <name> falls back to plain SSH

🤖 Generated with Claude Code

[github-actions]

⬇️ Go test coverage decreased from 26.2% to 25.4% compared to 7dddb76

⚠️ 1 of 10 packages have zero coverage.

• github.com/deevus/pixels

Updated Package Coverages:

# Package Name                                |  Prior |    New
- github.com/deevus/pixels/cmd                |   6.8% |   6.5%
+ github.com/deevus/pixels/internal/provision |  24.8% |  40.6%
- github.com/deevus/pixels/internal/ssh       |  29.0% |  18.4%
- github.com/deevus/pixels/internal/truenas   |  68.2% |  43.3%

View coverage for all packages

# Package Name                                | Coverage
- github.com/deevus/pixels                    |     0.0%
+ github.com/deevus/pixels/cmd                |     6.5%
+ github.com/deevus/pixels/internal/cache     |    93.8%
+ github.com/deevus/pixels/internal/config    |    94.0%
+ github.com/deevus/pixels/internal/dataset   |   100.0%
+ github.com/deevus/pixels/internal/egress    |    93.5%
+ github.com/deevus/pixels/internal/provision |    40.6%
+ github.com/deevus/pixels/internal/retry     |   100.0%
+ github.com/deevus/pixels/internal/ssh       |    18.4%
+ github.com/deevus/pixels/internal/truenas   |    43.3%