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>

agent/retry_utils.py

@@ -0,0 +1,57 @@
+"""Retry utilities — jittered backoff for decorrelated retries.
+
+Replaces fixed exponential backoff with jittered delays to prevent
+thundering-herd retry spikes when multiple sessions hit the same
+rate-limited provider concurrently.
+"""
+
+import random
+import threading
+import time
+
+# Monotonic counter for jitter seed uniqueness within the same process.
+# Protected by a lock to avoid race conditions in concurrent retry paths
+# (e.g. multiple gateway sessions retrying simultaneously).
+_jitter_counter = 0
+_jitter_lock = threading.Lock()
+
+
+def jittered_backoff(
+    attempt: int,
+    *,
+    base_delay: float = 5.0,
+    max_delay: float = 120.0,
+    jitter_ratio: float = 0.5,
+) -> float:
+    """Compute a jittered exponential backoff delay.
+
+    Args:
+        attempt: 1-based retry attempt number.
+        base_delay: Base delay in seconds for attempt 1.
+        max_delay: Maximum delay cap in seconds.
+        jitter_ratio: Fraction of computed delay to use as random jitter
+            range.  0.5 means jitter is uniform in [0, 0.5 * delay].
+
+    Returns:
+        Delay in seconds: min(base * 2^(attempt-1), max_delay) + jitter.
+
+    The jitter decorrelates concurrent retries so multiple sessions
+    hitting the same provider don't all retry at the same instant.
+    """
+    global _jitter_counter
+    with _jitter_lock:
+        _jitter_counter += 1
+        tick = _jitter_counter
+
+    exponent = max(0, attempt - 1)
+    if exponent >= 63 or base_delay <= 0:
+        delay = max_delay
+    else:
+        delay = min(base_delay * (2 ** exponent), max_delay)
+
+    # Seed from time + counter for decorrelation even with coarse clocks.
+    seed = (time.time_ns() ^ (tick * 0x9E3779B9)) & 0xFFFFFFFF
+    rng = random.Random(seed)
+    jitter = rng.uniform(0, jitter_ratio * delay)
+
+    return delay + jitter