docs(samples): backfill READMEs for 11 framework samples on agents branch

[mmercuri]

Summary

Backfills README.md for the 11 MINOR framework adapter samples on
feat/instrument-frameworks-agents (per A:/tmp/adapter-samples-audit.md):

• agno
• openai_agents
• llama_index
• google_adk
• bedrock_agents
• ms_agent_framework
• strands
• pydantic_ai
• embedding
• benchmark_import
• semantic_kernel

Every claim in every README is source-traced to the corresponding
samples/instrument/<adapter>/main.py. No aspirational text. All
READMEs follow the structure of the CLEAN reference samples
(samples/instrument/smolagents/README.md on the smolagents-closure
branch, samples/instrument/openai/README.md on the llm-providers
branch, samples/instrument/browser_use/README.md on the
browser-use-full branch).

Each README includes: title + 1-sentence summary, Prerequisites
(install extra + required/optional env vars), Run command (uv run python -m),
What this demonstrates (source-traced component table), Expected
output (every actual print path from main.py, including the
exit-code-2 short-circuits), and a Multi-tenancy note explaining the
current transitional state vis-a-vis PR #118.

Total addition: 826 LOC of doc — well under the 1500 LOC PR-size
guard, so this lands as a single PR.

Audit findings honored

• All 11 samples confirmed to have working main.py and no
  pre-existing README.md on feat/instrument-frameworks-agents
  (verified via git ls-tree -r origin/feat/instrument-frameworks-agents -- samples/instrument/).
• Install-extra names verified against pyproject.toml lines 40-51.
• The ms-agent-framework extra resolving to semantic-kernel is
  called out in the ms_agent_framework README (per the line-47
  pyproject mapping).
• embedding and benchmark-import empty extras are documented
  honestly — embedding notes that vector store hooks come from the
  underlying store, benchmark-import notes it ships no extra
  runtime deps.
• bedrock_agents and strands READMEs document live-only behavior
  (no mock mode in main.py) and the exit-code-2 short-circuit.
• benchmark_import README documents that it is a data importer
  (no HttpEventSink, no runtime events) — consistent with the
  audit row.

Multi-tenancy disclosure

Every README includes a "Multi-tenancy note" stating that the
adapter constructor does not yet accept org_id and that production
multi-tenant wiring lands with PR #118 (DRAFT). This is honest
disclosure of the transitional state, not a workaround.

Test plan

• Render-check each README on GitHub (markdown tables render, code blocks fenced correctly)
• Cross-check each "demonstrates X" claim against the linked main.py
• Confirm install-extra names in each README match pyproject.toml lines 40-51
• Confirm module-invocation paths (python -m samples.instrument.<adapter>.main) are canonical