M2: Extensions DI lifetime shape — transient disposable sub-client hazards (R2-1) #31

From the second verified review of PR #26 (finding R2-1, CONFIRMED). Decision-first: the registration shape needs a maintainer seal before more client families pile onto it.

Finding. AddOpenCode registers the typed OpenCodeClient transient over the factory client, and SessionsClient as a transient resolving a hidden OpenCodeClient per resolution (OpenCodeServiceCollectionExtensions.cs:53). Three lifetime hazards follow:

1. Root-provider retention — MS.Ext.DI tracks every transient IDisposable; a background service resolving from the root provider accumulates container-tracked OpenCodeClient instances until shutdown.
2. Scope-disposal poisoning — a consumer storing a scope-resolved SessionsClient beyond its scope gets ObjectDisposedException on every later call; SessionsClient is not IDisposable, so nothing signals the constraint, and Pipeline.Dispose sets _disposed = true unconditionally even on the borrowed-transport path where it owns nothing.
3. Split pipelines — injecting both OpenCodeClient and SessionsClient yields two unrelated pipelines; configuring/disposing one never affects the other.

Plus: the hand-grown one-line-per-family sub-client registration has no roster contract test, so the next generated client family silently lags DI until a consumer hits resolution failure.

Design space to weigh at the seal: scoped vs transient lifetimes; whether the borrowed-transport Pipeline.Dispose should no-op _disposed; resolving sub-clients from the same OpenCodeClient instance (e.g. via a scoped root); a generated or contract-tested registration roster (the Session 21 note already flags generator-emitted registrations as a trigger-based option).

Trigger: the first real DI consumer beyond the sandbox, or the next Extensions batch — whichever comes first. The hazards are live today but only bite consumers holding clients across scopes.

Milestone: M2 (the Extensions package grows in parallel with breadth; the shape must seal before more families multiply the registration list).