security: SSH host key verification with TOFU

[deevus]

Summary

• Replace StrictHostKeyChecking=no with accept-new mode using a pixels-managed ~/.config/pixels/known_hosts file — auto-accepts new hosts, rejects changed keys (MITM protection)
• SSH connections now use the container hostname (px-<name>) instead of IP to avoid stale known_hosts entries from IP reuse across container churn
• Lifecycle methods (Create, Start, RestoreSnapshot, Delete) clean up both hostname and IP entries from known_hosts

Test plan

• go build && go test ./... && go vet ./... all pass
• pixels create <name> creates a container and records its host key in ~/.config/pixels/known_hosts
• Subsequent pixels exec <name> -- hostname connects without prompts
• pixels destroy <name> removes the entry from known_hosts
• Opt-out: setting strict_host_keys = false under [ssh] in config falls back to old behavior

🤖 Generated with Claude Code

[github-actions]

⬇️ Go test coverage decreased from 30.3% to 25.1% compared to ec55944

⚠️ 1 of 10 packages have zero coverage.

• github.com/deevus/pixels

Updated Package Coverages:

# Package Name                                |  Prior |    New
- github.com/deevus/pixels/internal/config    |  94.4% |  49.6%
- github.com/deevus/pixels/internal/provision |  79.1% |  78.4%
- github.com/deevus/pixels/internal/ssh       |  34.7% |  25.9%
- github.com/deevus/pixels/sandbox/truenas    |  69.1% |  41.3%

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/config    |    49.6%
+ github.com/deevus/pixels/internal/egress    |    93.5%
+ github.com/deevus/pixels/internal/provision |    78.4%
+ github.com/deevus/pixels/internal/retry     |   100.0%
+ github.com/deevus/pixels/internal/ssh       |    25.9%
+ github.com/deevus/pixels/sandbox            |    83.3%
+ github.com/deevus/pixels/sandbox/incus      |     0.1%
+ github.com/deevus/pixels/sandbox/truenas    |    41.3%