[v1.0] Deliver a versioned extension SDK and testkit

[codeforester]

Goal

Make third-party extensions safe and practical to build without relying on private internals.

Scope

• Define versioned extension protocols, capability negotiation, lifecycle hooks, and error behavior.
• Provide a public testkit with fixtures for discovery, isolation, failure, and compatibility.
• Add a reference extension and a compatibility matrix.
• Document trust boundaries, permission expectations, and optional dependency behavior.
• Decide whether the SDK is part of base-cli or a separately versioned package.

Acceptance criteria

• An external contributor can build and test an extension using public APIs only.
• Incompatible extensions fail clearly without taking down unrelated extensions.
• SDK compatibility is tested across supported Python and Click/Typer versions.
• Security and lifecycle guarantees are documented.

Project fields

• Status: Backlog
• Priority: P1
• Area: Python
• Initiative: v1.0 Readiness
• Size: L