docs(#2276): 1.7.0 release documentation — what's-new, EoS explanation, feature index (#2282)

Add a curated 1.7.0 release-highlights page (docs/whats-new-1.7.0.md) and a
conceptual Embeddable Orchestration System (EoS) explanation
(docs/explanation/embeddable-orchestration-system.md), extend docs/FEATURES.md
with a v1.7.0 feature section, and wire both new docs into the docs index
(docs/README.md) and the root README.

Covers the release's marquee changes: the ADR-1239 Host-Integration Interface /
EoS (Embeddable Orchestration System) runtime expansion, the Capability + EoS
discoverability registries, the gsd-mcp-server companion, model-catalog advances
(GPT-5.6, (1M) badge), statusline enhancements, the compact GSD-state format,
plus a themed summary of the 100 fixes and 4 security hardenings.

Also corrects a stale CONTEXT.md glossary entry: the Capability Registry Overlay
now documents the #2009 fail-open behavior for a load-failed gate-declaring
capability (previously described as fail-closed).

American house style; no parity-gated reference docs hand-edited.

Refs #2276, #1678

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -60,6 +60,8 @@ New here? Follow [Your first project](docs/tutorials/your-first-project.md) for
 
 ## Documentation
 
+**What's new in 1.7.0** → [docs/whats-new-1.7.0.md](docs/whats-new-1.7.0.md)
+
 **Tutorials** — learning by doing:
 - [Your first project](docs/tutorials/your-first-project.md)
 - [Onboarding an existing codebase](docs/tutorials/onboarding-an-existing-codebase.md)