style: apply ruff lint --fix + ruff format across the tree

feat(exec): sandboxed per-session exec-kernel executor (ADR-0007 Phase 1)

feat(exec): extract Executor seam + EXECUTOR config switch (ADR-0007 Phase 0)

Security: replace pickle GPL cache with JSON; document accepted exec risk

Cap conversation history and code-output size to bound credit cost

Clean up dead code, fix two bugs, remove unused abstractions

Align project layout with HF Spaces and FastMCP conventions