feat: export/import workspace knowledge in .tgx bundles (#877 Phase 2) (#1024)

* feat: streaming N-Quads serializer for wire-format triples

Groundwork for Phase 2 of #877 (knowledge export). Hand-rolled
N-Triples term encoding: rdflib's term.n3() emits Turtle-style forms
(numeric shorthand, unescaped newlines) that are invalid in
line-oriented N-Quads, so literals are escaped per the ECHAR grammar
and IRIs validated for representability. Round-trip tests parse the
output back with rdflib's nquads parser and compare term-for-term.

* feat: export/import workspace knowledge in .tgx bundles (#877)

Phase 2 of the workspace bundle commands: tg-export-workspace now
includes the workspace's knowledge by default — per-collection
knowledge-graph triples as N-Quads (the collection names the graph,
streamed through a tempfile so memory stays flat regardless of
knowledge-base size) and the document library (metadata plus content,
fetched one document at a time). --config-only skips knowledge on both
sides; --triples-limit bounds very large graphs; -f/--flow-id selects
the flow the triples services run through.

tg-import-workspace streams triples back through the bulk import per
collection and recreates library documents (children after parents).
Knowledge import is additive, unlike config's skip-existing semantics.
Embedding vectors are not carried in bundles: --process re-runs
imported documents through the flow, which regenerates extraction
output and embeddings; --process-collection targets it.

Round-trip covered by unit tests over real archives: export with a
mocked Api, re-import, and assert the bulk triples stream and library
add calls reproduce the original values (including datatyped literals
via the N-Quads path).

tests/unit/test_cli/conftest.py

@@ -45,4 +45,18 @@ def sample_metadata():
         "metadata": [],
         "user": "test-user",
         "collection": "test-collection"
-    }
+    }
+
+def iri(v):
+    """Wire-format IRI term dict, as triples_query_stream yields them."""
+    return {"t": "i", "i": v}
+
+
+def lit(v, d=None, lang=None):
+    """Wire-format literal term dict (optional datatype / language)."""
+    t = {"t": "l", "v": v}
+    if d:
+        t["d"] = d
+    if lang:
+        t["l"] = lang
+    return t