Pick one HTTP client; remove the other

[franconicola]

Problem.
Both requests>=2.31.0 and httpx>=0.27.0 are required. Two HTTP stacks = two connection pools, two TLS contexts, two sets of retry semantics.

Actions.

• Recommendation: pick httpx (sync + async with one API).
• Inventory requests usage: rg -n 'import requests|from requests' hackagent/.
• Migrate call sites one by one. Watch out for requests.Session adapters and requests.exceptions.* catches.
• Remove requests from dependencies (keep transitively if needed).

Acceptance:
rg 'import requests' hackagent/ returns nothing; requests removed from pyproject.toml.