Remove em dashes where the replacement is unambiguous

The project has a standing style rule against em dashes that was missed
when these docs were written. This removes the 42 occurrences where the
substitution is mechanical and cannot change meaning:

- table cells using an em dash as a placeholder, now a hyphen
- em dashes immediately followed by a conjunction or subordinator
  (but, and, or, so, though, while, which, where, because, since),
  where a comma is simply correct

115 remain. Those join independent clauses, where a comma would create a
splice and a colon often reads wrong, so each needs a judgement call
rather than a pattern. A blanket regex over them was attempted and
reverted: it produced double colons and broken sentences.

docs/sustainability.md

@@ -177,7 +177,7 @@ Storage is **two tanks, not one**:
 
 A **combined ~250 m³ tank**, with the fire volume protected by taking the
 process suction from a higher level, is cheaper and has a real side benefit:
-process turnover stops the fire reserve going stagnant. Worth pricing — but
+process turnover stops the fire reserve going stagnant. Worth pricing, but
 only the fire design and the insurer can approve it, so the split above
 stands as the safe default until they do.