v0 stability and upgrades

AgentsKit Chat follows SemVer as a fixed package group.

• Patch releases preserve public TypeScript signatures, protocol versions, component/event meaning, and persisted session compatibility.
• During 0.x, a minor release may change a public contract, but only with an RFC or ADR as required, a changelog entry, a migration guide, and conformance evidence for all affected renderers.
• Protocol v1 payloads remain runtime validated. A breaking wire change uses a new protocol version and an explicit decoder/migration path.
• Deprecated APIs remain for at least one subsequent minor release unless a security issue makes that unsafe.
• AgentsKit peer minimums change only after upstream-first inspection and the full release matrix. No upstream implementation is copied downstream.

Applications should pin a minor line (^0.4.0 for this release), commit their lockfile, run renderer conformance and host E2E before upgrades, and review both the changelog and migration guide. Stable 0.1.x consumers should follow the 0.2 migration. Alpha GitHub tarball consumers should follow the alpha migration. Stable 0.2.x consumers should follow the 0.3 package consolidation migration. Applications already on 0.3.x can adopt 0.4.0 without changing their definition-owned sessions; controlled React and Ink modes are additive.