feat: v0.19.0 — debug-observability core complete

Milestone minor (operator-approved). Publishes the design-brief's
headline deliverable: the multi-pane debug-observability dashboard is
complete — all four observability panes are built and consuming their
real Worldtree endpoints:

- Persona      → GET /agents/{id}/persona_state
- Tools        → GET /sessions/{id}/tools
- BifrostState → GET /admin/sessions/{id}/bifrost
- AdminEvents  → GET /admin/events (SSE)

v1 client-REST coverage is 12/40; both non-REST surfaces (SSE 11/11,
Bifrost provider planes 8/8) already complete. Only Tier-2 client I/O
(transient-characters routing, persona_state-write) remains in scope;
everything else is covered or excluded-by-design in docs/coverage-map.md.

Version bump only (the feature arc landed across v0.18.5–v0.18.11).

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.18.11"
+version = "0.19.0"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"