[FEATURE]: Implement V2 manual compaction

Feature hasn't been suggested before.

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Describe the enhancement you want to request

SessionV2.compact is a stub that fails with OperationUnavailableError (packages/core/src/session.ts), which the server maps to a 503. So on V2 there is no manual compaction: /compact only appears to work in the app because the compat shim rewrites it to the legacy V1 session.summarize.

V2 already has automatic compaction (request-budget) and overflow-triggered compaction. What's missing is the explicit, user-initiated path.

This is already recorded as a follow-up in specs/v2/session.md:

Add explicit manual compaction on top of automatic request-budget compaction.

and in specs/v2/schema-changelog.md:

Defer provider-overflow recovery, explicit manual compaction, and deterministic old tool-result pruning.

Overflow recovery has since landed; manual compaction has not.

Notes on what makes it distinct from the automatic path:

• It should not be a provider turn — no inbox promotion, no tool materialization, no assistant reply.
• The two budget conditions shouldn't apply, since the request is explicit rather than a response to context pressure. In particular, automatic compaction declines when a model declares no limits.context, which would make /compact silently do nothing on such models.
• Automatic compaction retains compaction.keep.tokens of verbatim recent context because it only needs to free enough budget to continue. An explicit request should compress the whole active history.
• It needs to be serialized against that session's provider turns without joining an active drain or coalescing with another request.

No schema change is needed: SessionEvent.Compaction.{Started,Ended} and SessionMessage.Compaction already declare reason: "auto" | "manual", and "manual" currently has no producer outside tests.