feat: add release management commands

[eslerm]

Add release pipeline management: releases, pipelines, stages, and issue-to-release linking.

Note: Linear's Release API is marked ALPHA.

Scope

• 11 queries across 4 sub-entities (release, pipeline, stage, issue-to-release)
• 23 mutations including lifecycle ops (complete, sync) and access-key variants
• New command group with ~18 subcommands
• Full filter support
• Estimated ~3,000 LOC

Open Questions

• Implement ByAccessKey CI/CD variants? (different use case from CLI)
• Implement releaseSync? (integration-specific)
• Split into phases? (Phase 1: CRUD, Phase 2: pipelines/stages, Phase 3: issue linking)
• Should ALPHA features be gated behind a config flag?
• Is ALPHA status acceptable for shipping in a public CLI?

Depends on: SDK sync PR #48

See planning/04-releases.md