docs(readme): update assistant roster, prompt layers, repo structure

- Update assistant lists (added Shawn, Watson, David, CASE, AWS SA; modified Scotty/Harper roles)
- Reflect new architecture layers: Tool Prompt Snippets and Shared Context
- Align repository structure diagram with current filesystem layout

prompts/tools/argos.md

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+# Argos (web search + page fetch)
+
+Argos is your window onto the outside web.
+
+- Use Argos for the general web. For library/framework documentation, prefer Context7 — it returns better-structured results for that case.
+- For internal Agathos services, use Kernos, not Argos.
+- Quote queries when phrasing matters. Use search-engine operators when narrowing.
+- Cached search snippets can be stale. If "current state" matters (status pages, release notes), fetch the page itself rather than trusting the snippet.
+- For deep multi-query research, delegate to a research subagent rather than running long Argos chains in your own context.