gates: the newest release header must state its MinAgent; backfill v0.233.0-v0.236.0 (R-470, R-472)

From hub v0.112.0 a floor above the golden is served only with a declared
MinAgent, read from this CHANGELOG's newest `## vX.Y.Z` header. New fast,
blocking gate minagent_header_gate.py: that block must contain a line
starting `**MinAgent: X.Y.Z**`; prose, a code span or a non-bold mention
does not count (decoy test; red-proof F: relaxing the match to a body
mention fails test_decoy_body_mention_is_not_the_line).

Backfill: v0.233.0, v0.234.0, v0.235.0 and v0.236.0 carried no MinAgent
line. Each now reads `**MinAgent: 0.129.0** (unchanged)`, the value of the
nearest earlier header (v0.232.0). Proof it did not move: zero commits
under controller/internal/agentapi since 2026-09-01 (last: bb50e12,
2026-08-14), and the highest featureMinAgent entry is 0.129.0.

controller/scripts/test_controller_gates.py

@@ -32,6 +32,7 @@ FINGERPRINTS = [
     ("docker-v",       "docker -v gate OK"),
     ("debug-routes",   "debug route gate OK"),
     ("reuse-refs",     "cited paths — exact"),
+    ("minagent-header", "minagent header gate OK"),
 ]