#10276: fix(infra): use bidirectional jitter in computeBackoff

## Problem `computeBackoff` uses unidirectional jitter that only **adds** delay: ```typescript const jitter = base * policy.jitter * Math.random(); // [0, 1) → always positive ``` This reduces the effectiveness of jitter for preventing thundering herd, as all clients can only delay (not advance) their retries. ## Inconsistency The same codebase already implements correct bidirectional jitter in `src/infra/retry.ts`: ```typescript const offset = (Math.random() * 2 - 1) * jitter; // [-1, 1) → bidirectional ``` ## Solution Align `computeBackoff` with the existing `applyJitter` pattern: ```typescript const jitter = base * policy.jitter * (Math.random() * 2 - 1); return Math.min(policy.maxMs, Math.max(0, Math.round(base + jitter))); ``` With `jitter=0.2`: - **Before**: delay range `[base, base × 1.2]` (unidirectional) - **After**: delay range `[base × 0.8, base × 1.2]` (bidirectional) The added `Math.max(0, ...)` prevents negative delays when jitter is configured aggressively. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> - Updates `computeBackoff` (`src/infra/backoff.ts`) to use bidirectional jitter: the jitter term can now be negative or positive instead of always increasing delay. - Adds a lower clamp (`Math.max(0, ...)`) before applying `policy.maxMs`, preventing negative delays when jitter is configured aggressively. - Aligns backoff jitter behavior with the existing bidirectional jitter logic used elsewhere in infra retry utilities. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Change is small, localized to backoff jitter calculation, and now clamps negative delays to 0. Callers already clamp jitter to [0,1] in at least the primary config path, and the new behavior matches the existing bidirectional jitter pattern in infra retry utilities. - No files require special attention <!-- greptile_other_comments_section --> _{(5/5) You can turn off certain types of comments like style [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->