feat(skill-learning): preserve base skill during synthesis

app-instance/backend/tests/unit/test_skill_learning_synthesizer_preservation.py

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+from __future__ import annotations
+
+from beaver.memory.skills import SkillLearningCandidate
+from beaver.skills.learning.evidence import EvidencePacket
+from beaver.skills.learning.synthesizer import SkillDraftSynthesizer
+
+
+def test_revision_prompt_includes_base_skill_snapshot() -> None:
+    candidate = SkillLearningCandidate(
+        candidate_id="candidate-1",
+        kind="revise_skill",
+        source_run_ids=["run-1"],
+        source_session_ids=["session-1"],
+        related_skill_names=["debug-skill"],
+        reason="Improve debugging flow.",
+    )
+    packet = EvidencePacket(
+        run_ids=["run-1"],
+        session_ids=["session-1"],
+        task_summaries=["debug a failing test"],
+        session_excerpts=["assistant: fixed it"],
+    )
+    prompt = SkillDraftSynthesizer._build_prompt(
+        candidate,
+        packet,
+        "revise",
+        base_skill={
+            "skill_name": "debug-skill",
+            "version": "v0001",
+            "frontmatter": {"description": "Debug tests", "tools": ["read_file"]},
+            "content": "# Debug Skill\n\n## Safety\n\nDo not delete files.",
+            "summary": "Debug tests safely.",
+            "tool_hints": ["read_file"],
+        },
+    )
+
+    assert "Base skill snapshot" in prompt
+    assert "# Debug Skill" in prompt
+    assert "Do not delete files." in prompt
+    assert "preserved_sections" in prompt
+    assert "dropped_sections" in prompt