v0.12.0-rc.3 — read-only list had three writers in it

Supersedes 0.12.0-rc.2, which shipped a read-only command list with three
writing commands in it.

god-nodes, communities and related install the algo extension and build a
projected graph before they can rank anything — both are writes. related is
also an MCP tool, and the stdio server is read-only by design, so an assistant
calling it received a lock error from the engine.

It passed its tests because related returns early on an unknown seed id, and
every case written for it used one. The write path was never exercised.

Fixed, and the class with it: every entry in the read-only set is now executed
against a real read-only connection on a populated graph, with its keys taken
from the set itself. A writing command added there fails the suite.

Still the same thing to verify on Windows: run egr index . with Claude Code
open, then query. Two writers used to corrupt the graph; queries now open
read-only and coexist.