fix: reconcile speakers automatically by default

The plain transcribe command still used chunk overlap speaker matching,
so users had to discover a model flag or supply a known speaker count to
get the improved voice comparison behavior.

Enable OpenRouter voice comparison with automatic speaker counting for
normal text and SRT invocations. Keep --speakers and --speaker-model as
optional overrides, and let --no-diarization disable both provider labels
and voice comparison. Make the basic command the documentation entry point
and clarify the defaults in CLI help and repository guidance.

Cover bare invocations across one and multiple uploads, default SRT,
optional count constraints, opt-out ordering, and atomic output when the
default comparison fails.

Test Plan:
- python3 -B -m unittest -v: all 85 tests passed.
- bash -n transcribe: passed.
- git diff --cached --check: passed.
- Tests use real audio encoding with fake provider responses. Live voice
  accuracy remains unverified; no live API request was made for this change.

tests/fake_provider.py

@@ -35,6 +35,8 @@ def event(kind, **extra):
 event("start")
 if is_comparison:
     samples = json.loads((output.parent / "samples.json").read_text())
+    manifest = json.loads((staging / "chunks.json").read_text())
+    late_start = round(manifest[0]["end"]) if len(manifest) > 1 else 60
     content = request["messages"][-1]["content"]
     audio_parts = [part["input_audio"] for part in content if part["type"] == "input_audio"]
     assert len(content) == 2 * len(samples) == 2 * len(audio_parts)
@@ -73,7 +75,7 @@ if is_comparison:
     if mode == "reconcile-bad-json":
         output.write_text("not json")
         sys.exit(0)
-    assignments = [{"sample": sample["id"], "speaker": 0 if round(sample["start"]) in (1, 6, 60) else 1}
+    assignments = [{"sample": sample["id"], "speaker": 0 if round(sample["start"]) in (1, 6, late_start) else 1}
                    for sample in samples]
     if mode == "reconcile-missing":
         assignments.pop()
@@ -182,12 +184,13 @@ else:
     if mode.startswith("reconcile"):
         # Two labels already describe voice A inside the first request; later
         # chunks reuse the same local numbers for different request identities.
+        late_start = round(manifest[0]["end"]) if len(manifest) > 1 else 60
         words = [
             {"word": text, "start": start - chunk["start"], "end": start + 2 - chunk["start"],
              "speaker": speaker}
             for text, start, speaker in [
                 ("Alpha.", 1, 7), ("Again.", 6, 8), ("Beta.", 12, 9),
-                ("AlphaLate.", 60, 9), ("BetaLate.", 65, 7),
+                ("AlphaLate.", late_start, 9), ("BetaLate.", late_start + 5, 7),
             ] if start >= chunk["start"] and start + 2 <= chunk["end"]
         ]
     response = {"text": " ".join(w["word"] for w in words), "words": words}