fix(substantive_fixer): surface silent-skip reasons at INFO

Two silent paths in substantive_fix_cycle masked a 13-day stall:

1. Filter strips all candidates → return 0,0 with no log. With LIMIT 3
   ordered created_at ASC, if the oldest 3 have no fixer-actionable tags
   (e.g. eval_issues=[] from leo:skipped+domain:request_changes), the
   cycle silently picks the same head-of-line every tick.

2. _fix_pr early-returns logged at DEBUG only — invisible without
   fleet-wide DEBUG. Skip reasons (no_claim_files, no_review_comments,
   not_open lock, worktree_failed, etc.) never surfaced in journalctl.

Patch: log skipped candidate eval_issues when no actionable rows
found (path 1); promote DEBUG→INFO for per-PR skip reasons (path 2).
Zero behavior change — observability only.

Diagnosis context: 98 PRs stuck >3d, last successful substantive_fixer
event 2026-04-24. Need journal evidence to choose between (a) one-line
fix to the cycle, (b) larger _fix_pr regression. (Ship Step 2 directive.)

lib/substantive_fixer.py

@@ -539,6 +539,7 @@ async def substantive_fix_cycle(conn, max_workers=None) -> tuple[int, int]:
 
     # Filter to only PRs with substantive issues (not just mechanical)
     substantive_rows = []
+    skipped_no_tags = []
     for row in rows:
         try:
             issues = json.loads(row["eval_issues"] or "[]")
@@ -546,8 +547,20 @@ async def substantive_fix_cycle(conn, max_workers=None) -> tuple[int, int]:
             continue
         if set(issues) & (FIXABLE_TAGS | CONVERTIBLE_TAGS | UNFIXABLE_TAGS):
             substantive_rows.append(row)
+        else:
+            skipped_no_tags.append((row["number"], issues))
 
     if not substantive_rows:
+        # Visibility for the LIMIT-3 head-of-line block: if the oldest
+        # candidates have no fixer-actionable tags (e.g. eval_issues=[],
+        # broken_wiki_links only), the cycle silently returns 0 — and the
+        # next cycle picks the same head-of-line, forever. Log the eval_issues
+        # of skipped candidates so the journal makes the block visible.
+        if skipped_no_tags:
+            logger.info(
+                "Substantive fix cycle: 0 actionable from %d candidate(s) — head-of-line: %s",
+                len(rows), skipped_no_tags,
+            )
         return 0, 0
 
     fixed = 0
@@ -559,7 +572,13 @@ async def substantive_fix_cycle(conn, max_workers=None) -> tuple[int, int]:
             if result.get("action"):
                 fixed += 1
             elif result.get("skipped"):
-                logger.debug("PR #%d: substantive fix skipped: %s", row["number"], result.get("reason"))
+                # Was DEBUG — promoted to INFO to make stuck-PR root cause
+                # visible without enabling DEBUG fleet-wide. (Ship Apr 24+
+                # silent skip diagnosis.)
+                logger.info(
+                    "PR #%d: substantive fix skipped: %s",
+                    row["number"], result.get("reason"),
+                )
         except Exception:
             logger.exception("PR #%d: substantive fix failed", row["number"])
             errors += 1