docs(readme): add harness manager v0.9.0 SVG + callout

New diagram (docs/harness-manager.svg) shows the manifest-driven
adapter system: verb subcommands, manage TUI, Python backend, all 10
adapters wired to the shared .agent/ brain. Embedded in the main
README right after the existing architecture diagram with a v0.9.0
"what's new" blurb pointing at the changelog.

Author: codejunkie99

README.md

@@ -12,6 +12,20 @@ A portable `.agent/` folder (memory + skills + protocols) that plugs into Claude
   <img src="docs/diagram.svg" alt="agentic-stack architecture" width="880"/>
 </p>
 
+### New in v0.9.0 — harness manager
+
+<p align="center">
+  <img src="docs/harness-manager.svg" alt="harness manager v0.9.0" width="880"/>
+</p>
+
+Manifest-driven adapter system: every harness is now declared by an
+`adapter.json`, applied by a shared Python backend, and managed via
+verb subcommands or an interactive TUI. Cross-platform (POSIX +
+Windows) with concurrent-write protection, pre-v0.9 migration via
+`./install.sh doctor`, and shared-file ownership tracking so removing
+one adapter never orphans another. See [CHANGELOG.md](CHANGELOG.md)
+for the full list.
+
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