Resilience: reconnect shells and restore caller terminal state

[myobie]

Part of #21

What changed

• Preserve remote shell/exec work across transient reconnects with visible reconnect/resume notices.
• Treat explicit allow-list denial (code 403) as permanent refusal.
• Restore caller terminal state across shell completion, suspend/continue, and signal paths.

Validation

• cargo check
• cargo test --lib (138/138)
• cargo test --test local_slice (21/21)
• cargo test --test shell (5/5)
• git diff --check

Daemon sleep/wake self-heal remains in issue #21; physical PTY/sleep acceptance is deferred to field testing.