Add context activity backfill

Add context-auto, context-objects, and context-activities strategy
handling for context collection backfill.  The auto strategy classifies
ordinary objects directly and extracts objects from supported Create
activities while preserving shared request budgets, deduplication, abort,
and interval behavior.

Document the strategy behavior and add coverage for explicit, automatic,
empty, overlapping, and duplicate strategy configurations.

Assisted-by: Codex:gpt-5

Author: sij411

packages/backfill/README.md

@@ -61,3 +61,23 @@ for await (
 
 The seed object itself is not yielded.  If it appears in the discovered
 collection, it is skipped by ID.
+
+By default, `backfill()` uses the `context-auto` strategy.  In this mode,
+collection items are treated as backfillable objects by default.  If an item is
+recognized as an Activity, `backfill()` extracts the activity's object instead.
+
+To read only FEP-f228 activity collections, enable the `context-activities`
+strategy:
+
+~~~~ typescript
+for await (
+  const item of backfill({ documentLoader }, note, {
+    strategies: ["context-activities"],
+  })
+) {
+  console.log(item.object);
+}
+~~~~
+
+The `context-activities` strategy currently supports `Create` activities and
+yields the activity's object, not the activity itself.