v3.11.1 — README catch-up for v3.11.0 piston ship

Patch release. Docs-only. Three stale spots in README missed during
the v3.11.0 ship:

  - Tools and capabilities bullet list now includes piston
    (Sandboxed code execution — nsjail isolation, Python+Node
    default, PISTON_LANGUAGES extensible, REST + MCP).
  - Services table piston row: dead piston-pull sidecar reference
    removed; default langs corrected to Python+Node; pre-bake-at-
    build-time + aigate-internal-only network model documented;
    Authorization-header-strip detail added.
  - Security section: "All containers no-new-privileges" qualified
    for the two genuine carve-outs (piston privileged for nsjail,
    tailscale NET_ADMIN for tun) — matching the already-correct
    Network-isolation section lower in the README.

No code, config, or behavior changes.