perf(search): keyset pagination + fix the format-gate line-ending flip-flop

RECOMMENDATIONS #8: additive AfterSentAtUtc+AfterId cursor — browse windows fetch
via a (SentAtUtc, Id) keyset filter, O(pageSize) at any depth, COUNT skipped
(TotalCount=-1). Adds the Id tie-break to browse ordering (fixes latent
duplicate/skip on equal timestamps). Verified: InMemory continuation test + a
live-Postgres throwaway proving the Guid cursor comparison translates (removed).

Also root-causes the recurring pre-commit/CI format failures: dotnet-format on
Windows defaulted to CRLF while .gitattributes stores LF — the two fought forever.
Adds .editorconfig (end_of_line=lf etc.) so every tool agrees, and normalises the
straggler files. This bug bit three separate commits before being diagnosed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.editorconfig

@@ -0,0 +1,13 @@
+# Root editor/formatter config. end_of_line=lf makes dotnet-format agree with
+# .gitattributes (eol=lf) — without this, format-on-Windows wants CRLF while git
+# stores LF, and the pre-commit/CI format gates flip-flop forever.
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+
+[*.cs]
+indent_style = space
+indent_size = 4