core: import canonical plan JSON (on-hold)

[blindzero]

Goal

Enable tooling and support bundles by allowing a plan exported in canonical JSON to be imported back into an executable plan object.

Scope

• Add Import-IdlePlan (name TBD) that reads canonical JSON produced by Export-IdlePlan.
• Validate schema and fail with actionable errors on mismatch.
• Resulting plan object must be executable by the normal engine pipeline.

Non-Goals

• Not required for normal authoring flows (New-IdlePlan remains the primary plan creation mechanism).
• No migration of arbitrary user-authored JSON formats (only the canonical export format).

Acceptance Criteria

• Export -> Import roundtrip produces an equivalent plan (semantic equivalence).
• Invalid or non-canonical JSON fails with clear validation errors.
• Covered by unit tests.