Create CONTRIBUTING.md and STYLE_GUIDE.md

[osvaldoandrade]

Document contribution process and coding standards.

Goal: Guide external contributors.

CONTRIBUTING.md:

• How to set up dev environment
• How to run tests (make ci)
• PR process (squash, Conventional Commits)
• Code review expectations

STYLE_GUIDE.md:

• Go conventions (gofmt, goimports)
• Naming (single-word packages, no util/helper)
• Layer boundaries (reference ADRs)
• Testing (table-driven, testutil/wait)
• Error handling (typed errors)

Acceptance criteria:

• Both files created
• Referenced in README
• Align with ADRs

Part of: #655