docs(readme): mention the ACA provider

[Jason Robert (jrob5756)]

Summary

The README's provider list, comparison table, and docs index never mentioned the new experimental aca (Azure Container Apps) provider added in v0.1.26 (#338, closes #284). This adds it:

• Features bullet list mentions ACA sandboxed execution alongside the other providers.
• Providers comparison table gets an ACA column (tier, pricing, tool support, streaming, best-for).
• New "Using ACA (Experimental)" section with a minimal, schema-validated config snippet (provider.name: aca + required pool_endpoint + inner_provider), a summary of what it delivers, and a pointer to examples/aca-coding-agent.yaml.
• Documentation table links to docs/providers/aca.md.

Test plan

• Verified the README's provider: {name: aca, pool_endpoint: ..., inner_provider: copilot} snippet parses via ProviderSettings (schema validation passes; pool_endpoint is required for name=aca per src/conductor/config/schema.py).
• Documentation-only change; no code paths touched.