refactor: reapOrphanBlocks should live in acp-kernel, not the proxy

[ranxianglei]

Problem

reapOrphanBlocks (src/orphan-gc.ts) deactivates compression blocks whose effectiveMessageIds are no longer present in the visible context. This is block lifecycle / context-management logic — it belongs in acp-kernel, not the proxy adapter.

Currently the proxy:

• Iterates session.state.blocks (kernel-owned state)
• Tracks an orphanStreak counter on each block via a WeakMap<object, number> (attaching undocumented state to a foreign type)
• Calls deactivateBlock on kernel state

This violates the principle: the proxy is a pure adapter (format conversion + stream routing); all compression/context-management logic lives in acp-kernel.

Why it was put here

The kernel had no "reap orphan" capability. When a client (e.g. opencode) deletes history, a block's summary loses its anchor and floats to the top of context as an orphan, polluting the model's view.

Proposed fix

Move the capability into acp-kernel:

// in acp-kernel
core.reapOrphans(state: CompressionState, visible: CoreMessage[]): CompressionState

The kernel already owns state.blocks and effectiveMessageIds — it is the natural owner of this decision. The proxy reduces to a one-liner:

session.state = core.reapOrphans(session.state, visible);

Impact

• Does NOT affect prefix cache (only deactivates internal summaries; never touches bytes sent to provider).
• Pure architecture refactor — no behavior change.
• Not urgent; tracking here for when we next touch acp-kernel.

Related

Removed condenseOldToolResults (PR #11) for the same principle violation.