feat(ci): retry dep-PR container verification until publish lands

The post-merge dep-PR step races the producer's image-build workflow
and fails 404 before the tag is pushed. resolve_container_digest gains
a timeout_s retry (15s interval); --verify-timeout exposes it
(default 600s).

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docs/specs/DEVX-172.md

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+# DEVX-172: dep-PR retries container verification until publish lands
+
+## Problem
+
+Post-merge dep-PR runs concurrently with the producer's image-build
+workflow. `--verify-container` hits HTTP 404 before the push lands and
+skips PR creation — observed for sso-bridge v0.4.1 and v0.4.3.
+
+## Approach
+
+REQ-1: `resolve_container_digest(..., timeout_s)` retries a 404 lookup
+every 15s until the deadline.
+REQ-2: `--verify-timeout` CLI option (default 600s, 0 disables) wires the
+retry into the dep-PR step.
+
+## Test Plan
+
+- Unit test: 404-then-200 resolves the digest without raising.
+- Existing no-retry path (timeout_s=0) still fails immediately.
+
+## Deploy Plan
+
+devx release tag; producers inherit the 600s default on next pin bump.
+
+## Rollback Plan
+
+Revert; dep-PR verification fails fast on 404 again (status quo).
+
+## Acceptance Criteria
+
+- [x] REQ-1: 404 responses retry until `timeout_s` deadline.
+- [x] REQ-2: `--verify-timeout` option exposed, default 600.