procscope v0.2.0 — logsearch removed

logring.Handler in slog-configurator already is the slog.Handler and already
exposes Search / Count / Tail / Clear / Stats, so logsearch was wrapping a type
that needed no wrapper. Its filter type was already an alias of the ring's own,
so no query capability is lost.

Migration: logring.New(...) -> AddHandler(ring) -> ring.Search(...). The
extras logsearch added (page clamp, line truncation, count-before-paging)
belong on the ring and are not yet upstream; clamp and truncate at the call
site until they are.

The slog-configurator dependency goes with it. procscope now depends on
common-go and ctxerrors alone.

Also: pprofcapture gains a full README with runnable programs, and the root
README's log example -- which was missing its imports entirely -- is fixed.