docs: require context-compiler 0.7.0 in openwebui examples

[rlippmann]

What changed

• updated OpenWebUI pipe frontmatter requirements to the stabilized 0.7 package surface:
  • context-compiler>=0.7.0
  • context-compiler[experimental]>=0.7.0
• kept both OpenWebUI pipe frontmatter versions at 0.9.0
• updated OpenWebUI integration README requirement references from 0.6.14 to 0.7.0
• retained package-scoped compact trace import usage (context_compiler.observability.build_compact_trace_text) with no local compact-trace fallback logic

Why

• align OpenWebUI examples with the intended 0.7+ package boundary and avoid stale install guidance
• make requirement metadata consistent across frontmatter and README setup instructions

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)