docs: research Terraform HTTP delete and lock behavior

[nathanhruby]

Summary

• document Terraform HTTP backend behavior around destroy, state deletion, locking, and backend migration
• establish that ordinary terraform destroy writes empty state with the update method rather than invoking DELETE
• establish that DELETE and UNLOCK are separate operations and motivate idempotent post-delete unlock behavior

Supports Define logical-state deletion, retention, and path reuse in Plan United’s PocketBase-only replatform.

Branch note

This research branch was cut from the Wayfinder planning history, so the PR currently also contains the earlier CONTEXT.md planning commits. The research artifact itself is the tip commit 1caf4703.

Validation

• go test ./...
• mise run check:ci