feat: add OAuth provider adapters

[Zetazzz]

Summary

• Add protocol-neutral OAuth provider primitives and adapter contracts.
• Add Google and GitHub adapter implementations behind shared orchestration boundaries.
• Keep provider-specific exchange and identity normalization inside the adapters.

Stack

Base: #1703
Next: #1705

Validation

• packages/oauth tests: 15 passed
• packages/oauth build passed

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	jose@​5.10.0

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