feat(agent): add jittered retry backoff

Adds agent/retry_utils.py with jittered_backoff() — exponential backoff
with additive jitter to prevent thundering-herd retry spikes when
multiple gateway sessions hit the same rate-limited provider.

Replaces fixed exponential backoff at 4 call sites:
- run_agent.py: None-choices retry path (5s base, 120s cap)
- run_agent.py: API error retry path (2s base, 60s cap)
- trajectory_compressor.py: sync + async summarization retries

Thread-safe jitter counter with overflow guards ensures unique seeds
across concurrent retries.

Trimmed from original PR to keep only wired-in functionality.

Co-authored-by: martinp09 <martinp09@users.noreply.github.com>

tests/test_retry_utils.py

@@ -0,0 +1,117 @@
+"""Tests for agent.retry_utils jittered backoff."""
+
+import threading
+
+import agent.retry_utils as retry_utils
+from agent.retry_utils import jittered_backoff
+
+
+def test_backoff_is_exponential():
+    """Base delay should double each attempt (before jitter)."""
+    for attempt in (1, 2, 3, 4):
+        delays = [jittered_backoff(attempt, base_delay=5.0, max_delay=120.0, jitter_ratio=0.0) for _ in range(100)]
+        expected = min(5.0 * (2 ** (attempt - 1)), 120.0)
+        mean = sum(delays) / len(delays)
+        assert abs(mean - expected) < 0.01, f"attempt {attempt}: expected {expected}, got {mean}"
+
+
+def test_backoff_respects_max_delay():
+    """Even with high attempt numbers, delay should not exceed max_delay."""
+    for attempt in (10, 20, 100):
+        delay = jittered_backoff(attempt, base_delay=5.0, max_delay=60.0, jitter_ratio=0.0)
+        assert delay <= 60.0, f"attempt {attempt}: delay {delay} exceeds max 60s"
+
+
+def test_backoff_adds_jitter():
+    """With jitter enabled, delays should vary across calls."""
+    delays = [jittered_backoff(1, base_delay=10.0, max_delay=120.0, jitter_ratio=0.5) for _ in range(50)]
+    assert min(delays) != max(delays), "jitter should produce varying delays"
+    assert all(d >= 10.0 for d in delays), "jittered delay should be >= base delay"
+    assert all(d <= 15.0 for d in delays), "jittered delay should be bounded"
+
+
+def test_backoff_attempt_1_is_base():
+    """First attempt delay should equal base_delay (with no jitter)."""
+    delay = jittered_backoff(1, base_delay=3.0, max_delay=120.0, jitter_ratio=0.0)
+    assert delay == 3.0
+
+
+def test_backoff_with_zero_base_delay_returns_max():
+    """base_delay=0 should return max_delay (guard against busy-wait)."""
+    delay = jittered_backoff(1, base_delay=0.0, max_delay=60.0, jitter_ratio=0.0)
+    assert delay == 60.0
+
+
+def test_backoff_with_extreme_attempt_returns_max():
+    """Very large attempt numbers should not overflow and should return max_delay."""
+    delay = jittered_backoff(999, base_delay=5.0, max_delay=120.0, jitter_ratio=0.0)
+    assert delay == 120.0
+
+
+def test_backoff_negative_attempt_treated_as_one():
+    """Negative attempt should not crash and behaves like attempt=1."""
+    delay = jittered_backoff(-5, base_delay=10.0, max_delay=120.0, jitter_ratio=0.0)
+    assert delay == 10.0
+
+
+def test_backoff_thread_safety():
+    """Concurrent calls should generally produce different delays."""
+    results = []
+    barrier = threading.Barrier(8)
+
+    def _call_backoff():
+        barrier.wait()
+        results.append(jittered_backoff(1, base_delay=10.0, max_delay=120.0, jitter_ratio=0.5))
+
+    threads = [threading.Thread(target=_call_backoff) for _ in range(8)]
+    for t in threads:
+        t.start()
+    for t in threads:
+        t.join(timeout=5)
+
+    assert len(results) == 8
+    unique = len(set(results))
+    assert unique >= 6, f"Expected mostly unique delays, got {unique}/8 unique"
+
+
+def test_backoff_uses_locked_tick_for_seed(monkeypatch):
+    """Seed derivation should use per-call tick captured under lock."""
+    import time
+
+    monkeypatch.setattr(retry_utils, "_jitter_counter", 0)
+
+    recorded_seeds = []
+
+    class _RecordingRandom:
+        def __init__(self, seed):
+            recorded_seeds.append(seed)
+
+        def uniform(self, a, b):
+            return 0.0
+
+    monkeypatch.setattr(retry_utils.random, "Random", _RecordingRandom)
+
+    fixed_time_ns = 123456789
+
+    def _time_ns_wait_for_two_ticks():
+        deadline = time.time() + 2.0
+        while retry_utils._jitter_counter < 2 and time.time() < deadline:
+            time.sleep(0.001)
+        return fixed_time_ns
+
+    monkeypatch.setattr(retry_utils.time, "time_ns", _time_ns_wait_for_two_ticks)
+
+    barrier = threading.Barrier(2)
+
+    def _call():
+        barrier.wait()
+        jittered_backoff(1, base_delay=10.0, max_delay=120.0, jitter_ratio=0.5)
+
+    threads = [threading.Thread(target=_call) for _ in range(2)]
+    for t in threads:
+        t.start()
+    for t in threads:
+        t.join(timeout=5)
+
+    assert len(recorded_seeds) == 2
+    assert len(set(recorded_seeds)) == 2, f"Expected unique seeds, got {recorded_seeds}"