docs: add use_agent, graph, and elasticsearch_memory to README

[agent-of-mkmeral]

Summary

Adds documentation for three tools that were missing or incomplete in the README:

1. use_agent (Agent as Tool) — was completely missing
2. graph (Multi-Agent DAG) — was completely undocumented (distinct from agent_graph)
3. elasticsearch_memory — had usage examples but was missing from the Tools Overview table

Changes

1. Features Section

• Added 🤖 Agent as Tool bullet describing nested agent instances with model switching support
• Added 🔗 Multi-Agent Graph bullet describing deterministic DAG-based multi-agent pipelines

2. Tools Overview Table

• Added use_agent row with usage example and description
• Added graph row with usage example and description
• Added elasticsearch_memory row with usage example and description

3. Usage Examples Section

• Added comprehensive Use Agent (Agent as Tool) section with examples showing:
  • Basic usage inheriting parent agent's model
  • Model switching with different providers (bedrock, ollama, github)
  • Environment-based model selection (model_provider="env")
  • Custom model configuration with parameters
  • Tool filtering for nested agents
• Added Graph (Multi-Agent DAG) section with examples showing:
  • Creating a multi-agent research pipeline with per-node model configuration
  • Executing tasks through the graph
  • Status, list, and delete operations
  • Clear distinction from agent_graph (persistent message-passing vs DAG execution)

4. Environment Variables Section

• Added Use Agent Tool environment variables table
• Added Elasticsearch Memory Tool environment variables table (all ELASTICSEARCH_* vars)
• Added Graph Tool note about shared model env vars

Context

Requested by @mkmeral in this comment.

Ref: agent-of-mkmeral/strands-coder-private#64

[mkmeral]

@agent-of-mkmeral these are the other missing ones. can you add these too?

:x: Missing from README Tools Overview table entirely:
use_agent.py — spawn sub-agents dynamically
→ Just addressed by @murmeral in [PR #431](https://github.com/strands-agents/tools/pull/431) :tada:
graph.py — :warning: completely undocumented
→ Different from agent_graph.py (which IS in the table). graph.py has no entry anywhere in the README.
:warning: Partially documented (example section exists, but NOT in the Tools Overview table):
elasticsearch_memory.py — has a usage example at the bottom of README but is absent from the main tools table

[agent-of-mkmeral]

Done! Added both missing tools in commit 1019acd:

graph.py (was completely undocumented)

• ✅ Added to Features section: "🔗 Multi-Agent Graph"
• ✅ Added to Tools Overview table with usage example
• ✅ Added comprehensive Usage Examples section ("Graph - Multi-Agent DAG") with full DAG pipeline example showing create, execute, status, list, delete actions
• ✅ Added Environment Variables section (notes it shares model env vars with use_agent)
• ✅ Clearly distinguished from agent_graph (persistent message-passing vs deterministic DAG execution)

elasticsearch_memory.py (had examples but missing from table)

• ✅ Added to Tools Overview table with usage example
• ✅ Added Environment Variables section documenting all ELASTICSEARCH_* vars with defaults

Also replied to @hfurkanbozkurt's review thread clarifying the use_agent env vars.