fix(context): respect isolation scopes during compaction

[mikemikimike]

Closes #824

Problem

TokenBasedContextCompactor selected events from the entire session before applying workflow isolation. This allowed a peer node's event to reach the summarizer and then the current node's model after compaction.

Changes

• Filter active events by the invocation's isolationScope before token counting and compaction.
• Preserve the existing behavior for unscoped invocations.
• Add a regression test that verifies peer-scope events are excluded from the summarizer input.

Tests

• npx vitest run --project unit:core core/test/context/token_based_context_compactor_test.ts (8 passed)
• npm run ts:check (passed)
• npx eslint core/src/context/compaction_utils.ts core/src/context/token_based_context_compactor.ts core/test/context/token_based_context_compactor_test.ts (passed)
• npx prettier core/src/context/compaction_utils.ts core/src/context/token_based_context_compactor.ts core/test/context/token_based_context_compactor_test.ts --check (passed)
• git diff --check (passed)

Compatibility / Known limitations

Unscoped compaction remains unchanged. This PR does not add a live-model or end-to-end test; the regression is covered with deterministic unit-test fixtures and requires no credentials or external services.