feat(rag): harden Tier 1 retrieval observability and stability

- Add --rag-debug flag to show retrieved chunk names and similarity scores
- Add explicit fallback notices when RAG indexing/query embedding fails
- Log RAG index/query metrics (duration, scores, top hit, token estimate)
- Normalize and cap chunk content for more stable prompt shape on small models
- Add hypothesis-continuity instruction for follow-up prompts
- Add retrieval scoring API and new tests for truncation/fallback/debug paths

tests/test_cli.py

@@ -4,6 +4,7 @@ from typer.testing import CliRunner
 
 from tai.cli import app
 from tai.collectors import CollectedItem, CollectionReport
+from tai.rag_retriever import Chunk, EmbeddedChunk
 from tai.ssh_client import SSHCommandResult
 
 
@@ -230,3 +231,99 @@ def test_interactive_unknown_command_prints_hint(monkeypatch) -> None:  # type:
     assert result.exit_code == 0
     assert "AI Response" in result.stdout
     assert "Check logs." in result.stdout
+
+
+def test_interactive_prints_rag_fallback_notice_on_index_failure(monkeypatch) -> None:  # type: ignore[no-untyped-def]
+    _mock_session(monkeypatch)
+
+    async def fake_collect_from_plan(_session, _plan) -> CollectionReport:  # type: ignore[no-untyped-def]
+        return CollectionReport(
+            host="ssh.archflux.net",
+            items=[
+                CollectedItem(
+                    name="kernel",
+                    result=SSHCommandResult(
+                        command="uname -a",
+                        exit_code=0,
+                        stdout="Linux test",
+                        stderr="",
+                    ),
+                ),
+            ],
+        )
+
+    commands = iter(["what should I check next?", "/quit"])
+    monkeypatch.setattr("tai.cli.collect_from_plan", fake_collect_from_plan)
+    monkeypatch.setattr("tai.cli._try_embed_report", lambda *_args: (None, "embed failed", 1.0))
+    monkeypatch.setattr("tai.cli.AIClient.stream", lambda *_args, **_kwargs: iter(["Check logs."]))
+    monkeypatch.setattr("tai.cli.console.input", lambda _prompt: next(commands))
+
+    runner = CliRunner()
+    result = runner.invoke(
+        app,
+        [
+            "apache failed",
+            "--host",
+            "ssh.archflux.net",
+            "--port",
+            "5566",
+            "--no-probe",
+            "--interactive",
+        ],
+    )
+
+    assert result.exit_code == 0
+    assert "RAG unavailable (indexing failed)" in result.stdout
+    assert "AI Response" in result.stdout
+
+
+def test_interactive_rag_debug_prints_retrieval_scores(monkeypatch) -> None:  # type: ignore[no-untyped-def]
+    _mock_session(monkeypatch)
+
+    async def fake_collect_from_plan(_session, _plan) -> CollectionReport:  # type: ignore[no-untyped-def]
+        return CollectionReport(
+            host="ssh.archflux.net",
+            items=[
+                CollectedItem(
+                    name="kernel",
+                    result=SSHCommandResult(
+                        command="uname -a",
+                        exit_code=0,
+                        stdout="Linux test",
+                        stderr="",
+                    ),
+                ),
+            ],
+        )
+
+    commands = iter(["what should I check next?", "/quit"])
+    monkeypatch.setattr("tai.cli.collect_from_plan", fake_collect_from_plan)
+    monkeypatch.setattr(
+        "tai.cli._try_embed_report",
+        lambda *_args: (
+            [EmbeddedChunk(chunk=Chunk(name="kernel", content="content"), embedding=[1.0, 0.0])],
+            None,
+            1.0,
+        ),
+    )
+    monkeypatch.setattr("tai.cli.AIClient.embed", lambda *_args, **_kwargs: [1.0, 0.0])
+    monkeypatch.setattr("tai.cli.AIClient.stream", lambda *_args, **_kwargs: iter(["Check logs."]))
+    monkeypatch.setattr("tai.cli.console.input", lambda _prompt: next(commands))
+
+    runner = CliRunner()
+    result = runner.invoke(
+        app,
+        [
+            "apache failed",
+            "--host",
+            "ssh.archflux.net",
+            "--port",
+            "5566",
+            "--no-probe",
+            "--interactive",
+            "--rag-debug",
+        ],
+    )
+
+    assert result.exit_code == 0
+    assert "RAG retrieve:" in result.stdout