core: implement minimal plan/execute engine skeleton (step registry, events, validation)

[blindzero]

• Implements the initial headless IdLE engine flow: Request → Plan → Execute.
• Adds step dispatch via registry (supports scriptblock handlers for tests/hosts).
• Adds deterministic event stream and execution results.
• Enforces strict, data-only workflow validation (no ScriptBlocks).
• Adds declarative When conditions to skip steps.
• Adds docs + examples for quickstart and demo run.

Public API (expected):
Test-IdleWorkflow, New-IdleLifecycleRequest, New-IdlePlan, Invoke-IdlePlan

How to test:

Invoke-Pester -Path ./tests
pwsh ./examples/run-demo.ps1

Non-goals (explicitly not included):

• Provider implementations
• approvals
• secrets
• rollback orchestration
• deep-merge state