chore: add Prettier + lint-staged to monorepo (SPEC-006)

[kriss145]

Summary

• Proposes adding Prettier as the canonical formatter for the official-modules monorepo
• Adds lint-staged to auto-format staged files on commit
• Adds format:check CI step to fail PRs with inconsistent formatting
• Full design rationale in SPEC-006

Motivation

Currently each contributor formats TypeScript differently — spacing, quote style, semicolons, trailing commas. Three packages already show inconsistent style. As module count grows, divergence compounds and code reviews waste time on whitespace instead of logic.

Proposed config

{
  "printWidth": 120,
  "singleQuote": true,
  "semi": false,
  "trailingComma": "all"
}

Choices are aligned with the existing codebase majority style.

What this PR does NOT include

The implementation (actual Prettier install, bulk format commit, CI wiring) is intentionally left out. This PR opens the discussion — if maintainers agree with the approach, a follow-up PR will implement SPEC-006 Phase 1.

Migration note

Once the bulk format commit lands, all open branches will need a rebase. The spec proposes isolating the format commit and adding its SHA to .git-blame-ignore-revs to keep git blame readable.

Test plan

• Review SPEC-006 config decisions (printWidth, quotes, semi, trailingComma)
• Agree or propose changes to the Prettier config
• Confirm approach for git hooks runner (husky vs simple-git-hooks)
• Green-light implementation