Make workflow planning the default and add manual context compaction

[DevMando]

Summary

• Use the workflow planner for every generated plan and remove the legacy planner-engine configuration switch.
• Add /compact to summarize retained conversation context without removing the visible transcript; clarify that /clear fully wipes context.
• Update CLI help, planner documentation, and automated coverage. Remove stale v0.13.0 package release notes.

Why this matters

This completes the planner rollout for the next release: there is one supported planning experience rather than an optional mode. Users also have an explicit way to reduce context pressure while keeping their conversation visible.

Validation

• dotnet test tests/MandoCode.Tests/MandoCode.Tests.csproj --no-restore --filter FullyQualifiedNamePlannerEngineConfigTests|FullyQualifiedNamePlanRunnerBehaviorTests|FullyQualifiedName~InputStateMachineTests (60 passed on net8.0 and net10.0)