Drop Python 3.10, add Python 3.15 #3555

Python 3.10 reaches end of life in October 2026, and Python 3.15 is now released and rolling out to GitHub's runners. BBOT's supported range should move with that:

• requires-python in pyproject.toml: >=3.11,<3.16
• CI test matrix: 3.11 through 3.15
• Update the version range in AGENTS.md

Implemented in #3474.

Blockers

• Radixtarget blocking 3.15 support #3554: radixtarget's outdated pinned dependencies block installing on 3.15