fix: keep chunk details out of speaker headers

Render transcript-wide speaker numbers instead of exposing upload chunks in
reader-facing headers. Reconcile adjacent requests using at least two
unambiguous word-and-time matches in their shared audio, including when
provider speaker IDs change between requests. Reject conflicting mappings
and never treat matching numeric provider IDs as identity evidence.

Assign fresh IDs where continuity cannot be established and report that
uncertainty on stderr. Number only speakers retained after overlap trimming.
Document that overlap matching cannot identify a returning speaker who was
absent from the overlap, and add the reader-facing boundary to AGENTS.md.

Test Plan:
- python3 -B -m unittest -v: all 46 tests passed, including swapped IDs,
  ambiguous evidence, continuity across three requests, and plain headers.
- git diff --cached --check: passed.
- Tests use synthetic speaker labels; live diarization accuracy was not
  verified because provider credentials are unavailable in this environment.

test_transcript.py

@@ -22,13 +22,13 @@ class TranscriptTests(unittest.TestCase):
         ))
         self.assertEqual(response, original)
 
-    def test_speaker_ids_are_scoped_to_chunks_even_when_numbers_match(self):
+    def test_unmatched_speakers_get_separate_ids_without_chunk_headers(self):
         response = merge(
             [{"start": 0, "end": 63, "seam": 0}, {"start": 57, "end": 100, "seam": 60}],
             [{"words": [word("First.", 1, 0)]}, {"words": [word("Second.", 10, 0)]}],
         )
         self.assertEqual(render_text(response), (
-            "[Chunk 1, Speaker 0]\nFirst.\n\n[Chunk 2, Speaker 0]\nSecond.\n"
+            "[Speaker 0]\nFirst.\n\n[Speaker 1]\nSecond.\n"
         ))
 
     def test_long_unpunctuated_turn_is_wrapped_and_split_without_losing_words(self):