chore: cool rolling Pi patches for six hours

[quick-ricon]

Summary

• keep Sessions on the rolling v0.83.0-kkl Pi patch stream
• apply a Pi-specific six-hour minimum release age instead of mise's 24-hour default
• document the cooled patch-selection behavior in the generated README

Why

Fresh hosted agent installs selected Pi .3 for hours after .4 was released, while an explicit zero-age override selected .4. A six-hour per-tool policy preserves a cooling window without delaying internal KKL Pi patches for a full day or weakening the policy for unrelated tools.

Validation

• readme build --check
• codebase lint "$PWD"
• mise run lint:python
• mise run test (335 tests)
• cd cli && mix test (4 doctests, 132 tests)
• git diff --check
• project-scoped mise latest resolves Pi 0.83.0-kkl.4 while it remains below mise's default 24-hour age