docs: add YIP-88 (Governance overhaul, Part I)

[BobTheBuidler]

Summary

• Add YIP-88 Part I describing DAO operations and restructuring.

Rationale

• Provide the first part of the multi‑part governance overhaul in YIP format.

Details

• Includes the operational requirements and references section

[murderteeth]

Issues

• yip-88.md:6 - status: Proposed should be Approved. The governance forum confirms this YIP passed its Snapshot vote (Oct 23, 2025: "This YIP has passed").
• yip-88.md:5 - discussions-to URL contains placeholder slug yip-xx instead of yip-88: https://gov.yearn.fi/t/yip-xx-governance-overhaul-dao-restructuring/14553. While the URL resolves (Discourse uses the numeric ID), the slug should match the actual YIP number.

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This review was conducted using the review-pr skill.