fix: declare httpx as direct dependency

[minixalpha]

Summary

• Declare httpx>=0.25.0 as a direct runtime dependency.
• Update the lockfile so installations no longer rely on the anthropic transitive dependency.
• Record the installation fix in the 0.8.x changelog.

Motivation

nanoPyCodeAgent imports httpx directly for transport error handling. Declaring that dependency explicitly keeps clean and containerized installations correct even if the dependency graph of anthropic changes.

Validation

• uv lock --check --offline
• .venv/bin/python -m pytest — 129 passed
• Clean, uncached installation in the Terminal-Bench openssl-selfsigned-cert container image without --with httpx
• Verified installed package metadata includes httpx>=0.25.0, import nanopycodeagent succeeds, and the CLI starts successfully