docs(pi): name auto as the default routing mode

[tanglearncode]

The pi README still listed ask as the default routing mode:

- **ask** (default) — it proposes a switch and waits for you.
- **auto** — it switches on a clear match and tells you it did.
- **manual** — it never routes; `/neatcontext-use` only.

ask stopped being the default in #77, when the shortlist learned to ask
on its own — a near-tie became a question the route decides, so asking on
every turn bought nothing. #87 then made that default actually reach
machines whose plugin-routing.json had the old one written into it.

The docs never followed. This is the last place in the repo still
describing the old behavior — DEFAULT_MODE has been auto in
shared/core/routing.mjs throughout, and no other README, command, or
skill contradicts it.

After:

- **auto** (default) — it switches on a clear match and tells you it did, and
  asks first when two contexts are close.
- **ask** — it proposes a switch and waits for you.
- **manual** — it never routes; `/neatcontext-use` only.

auto leads the list, and the near-tie clause is spelled out — that
behavior is the reason ask no longer has to be everyone's default.

Docs only; no shipped source touched. npm test (494 pass) and
npm run check are clean locally.