Allow root signer rotation

[willjasen]

I've been working on a means to rotate the root signer, but it's not quite ready yet. The latest changes are at commit 738af05.

[willjasen]

Summary from the Copilot task:

Implemented so far:

- Implemented configuration-signing root rotation.

  - The current root can generate a replacement P-256 root.
  - Existing enrollment records are reissued under the new root.
  - Installations migrate their local signing records after validating the transition.
  - Root continuity and revoked enrollment protections remain enforced.

- Added compatibility for the original root-rotation format.

  - Early transition envelopes did not include the previous enrollment chain.
  - Those envelopes now have a narrowly scoped compatibility path.
  - New transitions still include and validate the complete previous chain.
  - Added regression coverage for the legacy transition format.

- Improved age unlock diagnostics.

  - The plugin now reports whether the Keychain entry is missing or why the stored identity failed to unlock the current configuration instead of showing only the generic age-ID prompt.