V1]individual] important email pinning

[clintjeff2]

Motivation

• Establish a clear, folder-local architecture contract for the V1 individual "Important Email Pinning" mini-product so future implementation can remain isolated and reviewable.
• Define ownership and responsibilities for components, services, hooks, tests, and docs so contributors know what may be changed locally and what requires a follow-up integration issue.
• Prevent accidental coupling to the main app (shell, routing, inbox, wallet, Stellar, DB, or shared design system) by documenting explicit integration constraints and a future adapter contract.

Description

• Added MODULE_BOUNDARIES.md to declare folder contract and explicit responsibilities for components, services, hooks, tests, and docs.
• Added DATA_OWNERSHIP.md to specify owned pin metadata, borrowed message-summary fields, derived data, persistence assumptions, and data-protection rules.
• Added INTEGRATION_CONSTRAINTS.md and updated README.md, specs.md, and REVIEW_NOTES.md to reference the architecture deliverables and to make prohibited host-app dependencies and required future adapter behavior explicit.

Testing

• Ran local validation checks (git diff --check) which completed without errors.
• Verified repository state with git status --short which reported the expected changes.
• No other automated tests were required for this documentation-only change.

---

Closes #369