Clarify or change sessions list --all limit behavior

[quick-ricon]

sessions list --all --json still applies the default --limit 20, which is surprising when using it for corpus counts.

Observed on Or's machine:

sessions list --all --json | jq length
# 20

sessions query --limit 10000 --sql 'select count(*) as recorded_sessions from sessions' --format json
# 659

--all reads like "all sessions" but currently means "include empty/stub sessions within the current limit". That is easy to misuse for README/status metrics.

Possible fixes:

• make --all imply no limit unless --limit is explicitly supplied; or
• keep behavior but rename/clarify help text, e.g. "include empty/stub sessions hidden by default (still limited; default 20)".