feat: restore last used ctx and ns

[pasteley]

Add support for restoring the last-used context across shell sessions:

Closes: #88

What's added

• kubie ctx --last-used cli argument: attempts to restore the most recently used context from kubie state.
• behavior.track_last_used config option: when enabled, kubie records the last successfully entered context into the state file on every successful context entry.

Motiviation

Automatically re-enter context used most recently (without interacting with the context selector) on new terminal sessions (e.g. zsh init).

This PR provides:

• explicit restore via --last-used (safe to call from shell init)
• opt-in recording via behavior.track_last_context (keeps default behavior unchanged)

How it works

Recording

When behavior.track_last_used: true, kubie updates state on each successful kubie ctx ... entry:

• state.last_context = <resolved context name>

This happens regardless of whether the context was chosen via selector or passed positionally.

Restoring

When kubie ctx --last-used is called:

• kubie reads state.last_context
• if it exists and the context is still installed → enter it
• if it is missing/invalid → no-op (exit 0, no selector UI), making it safe for shell init

kubie ctx --last-used <context> behaves like kubie ctx <context> (explicit context wins); the flag only affects whether restore-from-state is attempted.

Typical usage (in shell init)

if [[ -o interactive ]] && [[ -z "${KUBIE_ACTIVE}" ]]; then
  kubie ctx --last-used >/dev/null 2>&1
fi