Fix 0086 changelog em dash and stale ttl message

Address CoPilot review on #227:

- CHANGELOG 0.17.0 header uses a hyphen, not an em dash, per
  .github/copilot-instructions.md (new entries avoid the em dash;
  older headers are intentionally left as-is).
- The fetch negative-ttl ValueError message no longer says "None
  preserves current behavior"; with a manager default configured,
  per-call None selects the default. Reworded to match the precedence.

Comment/wording only; no behavior change.

Author: chris-colinsky

CHANGELOG.md

@@ -4,7 +4,7 @@ All notable changes to `openarmature-python` are documented in this file.
 
 The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/). The package follows [Semantic Versioning](https://semver.org/); pre-1.0 minor bumps may carry behavioral changes per [spec governance](https://github.com/LunarCommand/openarmature-spec/blob/main/GOVERNANCE.md).
 
-## [0.17.0] — 2026-07-20
+## [0.17.0] - 2026-07-20
 
 ### Added
 

src/openarmature/prompts/manager.py

@@ -150,7 +150,8 @@ async def fetch(
         if cache_ttl_seconds is not None and cache_ttl_seconds < 0:
             raise ValueError(
                 f"cache_ttl_seconds must be >= 0 (got {cache_ttl_seconds!r}); "
-                "None preserves current behavior, 0 forces a fresh read"
+                "None selects the manager default or the backend's own caching, "
+                "0 forces a fresh read"
             )
         # Proposal 0086 precedence: an explicit per-call value (including
         # 0) wins; else the manager default; else None (the backend's own