docs(context): define exploration checkpoint and rewind boundaries

[627150795]

Problem

OpenPI has a context_pivot capability, but there is no explicit design contract distinguishing recyclable exploration checkpoints and rewind from Session compaction, branch navigation, or filesystem rollback.

Value

This gives contributors a safe vocabulary for recovery boundaries, branch preservation, owner responsibilities, artifact evidence, retention, and failure states without promising that provider state or external side effects can be undone.

Approach

Added a focused design document and linked it from the design index. The proposal keeps rewind append-only and branch-based, treats worktree restoration as a separate future permissioned operation, and keeps Pi as the owner of Session history and compaction.

Validation

• git diff --cached --check
• bun run lint
• bun run typecheck

Impact

Documentation-only; no runtime, API, Git-ref, filesystem, or persisted-data behavior changes.

Fixes #154

[tt-a1i]

Thank you for preserving this research. We are closing this PR as a source record for the Execution lifecycle consolidation tracked in #277. This is not a finding that the content is wrong. The current queue contains overlapping contracts from one stale base, so we will extract only settled facts and unresolved decisions into at most one successor record, citing this PR explicitly. Please do not spend time rebasing this branch.