Teach the indexing-diagnostics skill the new search-timing logs (Mode G)

Adds Mode G — "an in-render `_search` was slow (server-side search timing)" —
documenting the `realm:search-timing`, `realm:requests` (`dur=`), and
`realm:health` log channels keyed by the `x-boxel-logging-correlation-id`
correlation id: the line formats, a stage-by-stage decision tree (incl. the
`dur >> handler` + event-loop-lag saturation fingerprint and `coalescedWait`),
how to pull them from CloudWatch / Loki, and the limits (prerender-gated, not
in `boxel_index.diagnostics`). Cross-referenced from the data-stall category,
the diagnostics-locations list, and Mode D so a render stalled in
`waiting-stability` on a `_search` routes here.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: habdelra