feat(sourcehunt): make sandbox memory configurable with 4 GiB default

[jorge-garcia-le]

Summary

• Set the SourceHunt sandbox memory default to 4096 MiB in deep and constrained modes.
• Add --sandbox-memory-mb and carry the setting through structured config, commit monitoring, and proof runs.
• Validate limits from 512 to 16384 MiB and cover defaults, overrides, and CLI wiring.

Verification

• Focused sandbox, proof, and commit monitor tests: 65 passed.
• Broader SourceHunt and sandbox regression tests: 127 passed.
• Ruff lint and format checks pass for all changed files; git diff --check passes.
• Full strict suite: 3767 passed, 1 skipped, 3 failed in provider routing and telemetry tests outside this patch (test_machine_cli, test_minimax_compat, test_otel). Repository-wide Ruff lint and format checks also report existing violations outside the changed files.