fix(tui_gateway): clamp -1 post-compression sentinel in context_used

The salvaged fix guards with `if ctx_max and last_prompt`, but last_prompt
comes from `last_prompt_tokens or 0` — the post-compression -1 sentinel
(conversation_compression) is truthy, so it leaked context_used=-1 on the
transitional turn. Clamp <0 to 0 so it reads as unknown (no gauge), matching
the CLI status-bar path (cli.py _get_status_bar_snapshot).

Follow-up on the salvaged #50518 (r266-tech).

tests/test_tui_gateway_server.py

@@ -8513,3 +8513,22 @@ def test_get_usage_reports_real_current_occupancy():
     assert usage["context_used"] == 60_000
     assert usage["context_max"] == 120_000
     assert usage["context_percent"] == 50
+
+
+def test_get_usage_clamps_post_compression_sentinel():
+    """Right after a compression, last_prompt_tokens is the -1 sentinel
+    (conversation_compression sets it until the next real usage report). It is
+    truthy, so `or 0` doesn't neutralize it — the guard must clamp <0 to 0 so
+    the transitional turn emits no gauge instead of leaking context_used=-1."""
+    agent = types.SimpleNamespace(
+        model="test-model",
+        session_total_tokens=4_000_000,
+        context_compressor=types.SimpleNamespace(
+            last_prompt_tokens=-1,
+            context_length=1_048_576,
+            compression_count=6,
+        ),
+    )
+    usage = server._get_usage(agent)
+    assert "context_used" not in usage
+    assert "context_percent" not in usage

tui_gateway/server.py

@@ -2977,7 +2977,13 @@ def _get_usage(agent) -> dict:
         # fabricated 0% or the old cumulative reading. The built-in compressor
         # always reports a real last_prompt_tokens once a turn runs, so it is
         # unaffected.
+        # Clamp the -1 "compression just ran, awaiting real usage" sentinel
+        # (conversation_compression.py) to 0 so the transitional turn reads as
+        # unknown (no gauge) instead of leaking context_used=-1. Matches the
+        # CLI status-bar path (cli.py _get_status_bar_snapshot).
         last_prompt = getattr(comp, "last_prompt_tokens", 0) or 0
+        if last_prompt < 0:
+            last_prompt = 0
         ctx_max = getattr(comp, "context_length", 0) or 0
         if ctx_max and last_prompt:
             usage["context_used"] = last_prompt