docs(cache): define prompt cache ownership and lifecycle contract

[627150795]

Problem

Prompt-cache ownership, identity, accounting, and lifecycle boundaries are easy to conflate with Session context, memory, compaction, pruning, summarization, or replay.

Value

This gives contributors and maintainers one explicit contract for what OpenPI, Pi, providers, and operators can claim, including cache evidence and invalidation limits.

Approach

Added a focused design document and linked it from the design index. The contract keeps provider cache state in the provider/client layer and does not introduce a cache registry or runtime behavior.

Validation

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

Impact

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

Fixes #191

[tt-a1i]

Thank you for preserving this research. We are closing this PR as a source record for the Context and model invocation 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, recommendations, and unknowns into at most one successor record, citing this PR explicitly. Please do not spend time rebasing this branch.