docs(context): define model-visible context contract

[627150795]

Problem

Model-visible context sources, injection order, provenance, authority, and provider-boundary evidence are currently spread across implementation notes and related issues.

Value

This contract gives contributors a shared inventory of 18 context source classes and clearly separates placement, provenance, runtime authority, provider serialization, and model attention.

Approach

• Added a focused OpenPI/Pi context contract covering system construction, hooks, repository context, Trust, Skills, tools, deferred discovery, MCP boundaries, compaction, branches, and child Sessions.
• Documented discovery/load/refresh/cost classes, intermediate versus final evidence, sensitive observability limits, and bounded comparisons with Codex, Hermes, Grok Build, Gemini CLI, OpenCode, and Claude Code.
• Preserved related issue ownership and explicitly avoided a second prompt assembler, MCP stack, memory store, or resource 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, tool behavior, storage behavior, or provider serialization changed.

Impact

Documentation-only. The contract records current behavior and evidence limits; implementation remains owned by Pi and the related extension issues.

Fixes #194

[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.