Add independent release automation + surface gateway spec version

[njbrake]

Note: this issue was drafted by Claude via back-and-forth with @njbrake. The reasoning and decisions are his; the prose is Claude's.

Part of the SDK release coordination defined in mozilla-ai/otari#133. This SDK is released independently of the gateway, on its own semver.

• Add release automation (release-please) that proposes a version-bump PR from conventional commits and publishes on merge.
• Publish step targets PyPI.
• Surface the gateway spec version baked into the generated core (e.g. __spec_version__).
• Confirm the endpoint-coverage drift gate flags a regen PR before release, and that the hand-written ergonomic alias layer covers any new control-plane endpoint.

Upstream policy and rationale: mozilla-ai/otari#133.