[agent-harness] P4.2 Document .cursor/plans as task state vs chat

[ManintheCrowds]

Phase 1 - P4.2

Depends on: #3

Outcome: Document .cursor/plans as task state vs conversational transcript; link OpenHarness handoff docs where applicable.

Acceptance criteria

• P4.1 ([agent-harness] P4.1 ADR - idempotent vs non-idempotent agent flows #3) merged.
• Harness or MiscRepos doc cross-link (path documented for sibling clone).

[ManintheCrowds]

Repo naming: Canonical policy and checklist — docs/engineering/OPENGRIMOIRE_NAMING_AND_URLS.md (GitHub ManintheCrowds/OpenGrimoire, local folder OpenGrimoire).