docs(routing): define provider and model contract

[627150795]

Problem

Provider/model selection, routing, fallback, reasoning, and invocation evidence are currently described across separate notes without one shared contract.

Value

This gives contributors one code-grounded vocabulary for configured, selected, resolved, routed, attempted, provider-returned, and successful states, while keeping Pi and OpenPI ownership boundaries explicit.

Approach

• Added a documentation-only provider and model routing contract.
• Defined retry, fallback, reasoning, identity, replay, and evidence boundaries without adding a second runtime registry or router.
• Added the contract to the design documentation index.

Validation

• git diff --check — passed
• bun run lint — passed
• bun run typecheck — passed
• No runtime code, setup behavior, or configuration semantics changed.

Impact

Documentation-only. The contract records current boundaries and unknowns; it does not change execution behavior.

Fixes #195

[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.