Add protocol version negotiation to agent/initialize

[aniketwattamwar]

Problem

There is no protocol version anywhere in the wire format. agent/initialize exchanges a name and an empty capabilities object, so a runtime cannot tell whether it is talking to an agent built against today's spec or one from six months ago. There is no safe way to ever make a breaking change.

Proposal

Both directions carry protocol_version, a MAJOR.MINOR string, deliberately decoupled from the package version.

• Runtime sends the highest version it speaks
• Agent responds with the version it will actually speak
• Differing major → runtime aborts with a clear error
• Differing minor → both operate at the lower minor
• Absence means 0.1 — every agent built against the current SDK keeps working

Acceptance criteria

• protocol_version in initialize params and result
• Legacy agents (no field) still run, with a single warning
• Major mismatch aborts with error -32001, not a confusing failure
• Spec, docs/spec.md, and schemas updated

See ECP-0001 phase 1. This is the only roadmap item with a closing window — it gets harder with every agent built against a versionless protocol.