feat(api): manifest formatter with token-budget truncation

tests/test_manifest_injection.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+from app.core.deep_agent import format_folder_manifest, MANIFEST_TOKEN_BUDGET
+
+
+def test_format_folder_manifest_basic():
+    manifest = {
+        "folderPath": "D:\\Acme",
+        "lastScannedAt": "2h ago",
+        "files": [
+            {"relPath": "briefs/kickoff.md", "kind": "text", "summary": "Kickoff notes; scope and deadlines."},
+            {"relPath": "logos/logo-v3.png", "kind": "image", "summary": "Final logo on white."},
+        ],
+    }
+    out = format_folder_manifest(manifest)
+    assert "<linked_folder>" in out
+    assert "/briefs/kickoff.md" in out or "briefs/kickoff.md" in out
+    assert "[text]" in out
+    assert "[image]" in out
+
+
+def test_format_folder_manifest_truncates_past_budget():
+    files = [
+        {"relPath": f"f{i}.md", "kind": "text", "summary": "x" * 100, "mtimeMs": i}
+        for i in range(2000)
+    ]
+    out = format_folder_manifest({"folderPath": "p", "lastScannedAt": "now", "files": files})
+    assert "more files omitted" in out
+    # Rough token check
+    assert len(out) // 4 < MANIFEST_TOKEN_BUDGET + 200
+
+
+def test_format_folder_manifest_null_returns_empty():
+    assert format_folder_manifest(None) == ""
+    assert format_folder_manifest({"files": []}) == ""