Make reset restore a reinstallable direct agent binary

[bcho]

Description

aks-flex-node reset removes the agent service and node runtime state but intentionally retains the managed blue/green binary layout:

• /usr/local/bin/aks-flex-node remains a symlink to aks-flex-node-current.
• /usr/local/lib/aks-flex-node retains the current, last-good, blue, and green paths.

This makes a clean reinstall ambiguous. scripts/install.sh must either reject the retained managed link or special-case a reset host by deleting the inactive layout before installing a direct binary. It also caused the rejoin path exercised by PR #308 to invoke the installer against a retained managed link.

Expected behavior

After reset, the agent should remain callable but be in a state that supports a clean installer retry. Consider having reset atomically restore the active agent as a regular /usr/local/bin/aks-flex-node file and remove the managed layout after the service has stopped.

This would preserve the aks-flex-node command while making the post-reset state equivalent to a pre-managed installation. Normal in-place upgrades while the service is installed should continue to use AgentUpgrade.

Acceptance criteria

• aks-flex-node reset leaves an executable agent command available.
• /usr/local/bin/aks-flex-node is no longer a managed-layout symlink after a successful reset.
• The inactive managed blue/green layout is removed safely.
• Running scripts/install.sh after reset succeeds without installer-specific managed-layout cleanup.
• Reset remains idempotent and has coverage for direct, managed, and already-reset binary layouts.

Context

Follow-up to #307 and PR #308.