fix(content): PR #61 round 2 — schema fields, cooldown, CHANGELOG sections

Contradiction files:
- Add role/access/trust back to all lines (schema-required)
- prerequisite: → prerequisites: (plural, matching schema)
- Remove cooldown entirely — fire-once enforced by trigger semantics
  (contradiction_detected fires once per KG state change), not magic numbers

Tutorial files:
- Remove cooldown: -1 — same reasoning, first_* triggers are one-time

Server:
- Document cooldown: Option<i32> on MonologueLine — None means no
  per-line cooldown, fire-once relies on trigger design

CHANGELOG:
- Move displaced entries from ### Fixed back to ### Changed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/content/types.rs

@@ -517,6 +517,9 @@ pub struct MonologueLine {
     pub prerequisites: Option<Prerequisites>,
     #[serde(default)]
     pub priority: Option<i32>,
+    /// Per-line cooldown in ticks. `None` (omitted in YAML) means no per-line
+    /// cooldown — fire-once lines rely on trigger semantics instead (e.g.,
+    /// `first_*` and `contradiction_detected` triggers fire once by design).
     #[serde(default)]
     pub cooldown: Option<i32>,
     #[serde(default)]