docs: Document agent modernization architecture

[theinterneti]

Summary

Document the new agent architecture where:

• .agent.md files define agent personas and Copilot interaction
• .instructions.md files provide atomic rules auto-injected by glob
• Skills provide curated in-context reference material
• OpenHandsPrimitive provides programmatic execution via OpenHands headless mode
• OpenHands presets configure tool sets and system prompts per agent type

Deliverables

1. docs/agents/dev/agent-modernization.md — Architecture decision record (ADR):

   • Why the Python agent system was archived
   • How OpenHands replaces it
   • Migration guide for any code referencing old agents

2. Update docs/agents/dev/composition-model.md — Add execution layer section showing how OpenHands presets connect to the composition model

3. Update PRIMITIVES_CATALOG.md — Deprecation notices on archived agent primitives, new preset entries

4. Update routing rows in AGENTS.md if needed

Acceptance criteria

• ADR written and linked from AGENTS.md
• Composition model doc updated with execution layer
• PRIMITIVES_CATALOG.md updated
• No stale references to archived agent classes in docs

[github-actions]

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