Add the GitHub Extension 1.0 spec

[Michael Jolley (michaeljolley)]

The Preview has grown past its saved query roots, and the backlog needs one product and architecture contract before implementation heads in a dozen directions.

The plan

• Define the 1.0 hub, repository surfaces, primary actions, and explicit boundaries.
• Set the authentication, account, storage, cache, API, migration, telemetry, accessibility, and release contracts.
• Map the open backlog and active pull requests to the proposed delivery order.

This is a spec only. It does not change runtime behavior.

[Copilot]

Pull request overview

Adds a new written specification document for “GitHub Extension 1.0” to establish a shared product + architecture contract before implementation work expands beyond the current saved-query focus.

Changes:

• Introduces a 1.0 product definition (hub + repository surfaces), feature scope, and explicit out-of-scope boundaries.
• Defines architecture-level contracts for authentication/accounts, data persistence, API/service layering, async loading, error model, telemetry, accessibility, and security.
• Documents a proposed delivery order, release gates, and maps the existing backlog/PRs to that plan.

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