Merge CB-602: guard against a config key that never reaches the example

bridged.yaml is gitignored, so bridged.example.yaml is the only
committed description of the config schema. Two tests already covered
example -> code; nothing covered code -> example, so a brand-new key
could ship undocumented and no test would notice.

A new test compares BridgedConfig.KNOWN_TOP_LEVEL_KEYS against the
example scanned as TEXT, so a key documented only as a comment counts as
documented. That is what makes the guard correct rather than annoying:
most of the example is commented on purpose.

Verified here: added an undocumented key and watched the test fail with
an actionable message naming it; then documented that key as a comment
only and watched it pass. Probe reverted, tree clean.

Closes #96

bridged/src/main/java/dev/ltms/bridged/config/BridgedConfig.java

@@ -967,8 +967,12 @@ public record BridgedConfig(
     /**
      * Top-level keys this version understands. Used only to warn about the rest — see
      * {@link #warnUnknownTopLevelKeys}. Keep in step with the record components.
+     *
+     * <p>Package-private (not {@code private}) so a test can assert every key here is documented in
+     * {@code bridged.example.yaml} — the only committed description of the config schema, since
+     * {@code bridged.yaml} itself is gitignored.
      */
-    private static final Set<String> KNOWN_TOP_LEVEL_KEYS = Set.of(
+    static final Set<String> KNOWN_TOP_LEVEL_KEYS = Set.of(
             "bind", "herdrSocket", "profiles", "guard", "worktreeRoot",
             "lifecycle", "spawnReadyTimeoutMs", "spawnReadyPollMs", "broker", "primary", "fleet",
             "leadHeartbeat", "health", "placement", "auth", "configReload", "quarantineCooldownSeconds");