v0.9.14: session-bound persistent execution kernel (evolve code_execution, no second runtime)

Source: tool-surface survey 2026-09-15. `code_execution` (Python) and `js_execution` (Node) exist but spawn a fresh process per call — no state, slow for iterative data work.

The machinery for a persistent kernel already exists and is hidden: `rlm` was exactly that and is now compat-only; the hidden `repl` kernel superseded it.

## Proposal

Resurface a minimal session-bound persistent-kernel variant — or make `code_execution` optionally session-bound — serving the same data story as notebook support. **Constraint:** this must stay within the one-runtime principle — evolve the existing tool, do not add a second execution authority.

Start by tracing the existing code_execution and js_execution entry points, then compare the hidden rlm and repl kernel behavior. Define the smallest session-bound evolution of an existing tool, with completion requiring persistent state for iterative data work while preserving the one-runtime principle and adding no second execution authority.

Written by the indexing model from the issue text.