Add count mode to logs command

[lntutor]

Summary

• keep -n for limiting displayed logs and rename its long form to --limit
• add llm logs --count using the existing query construction so all filters are respected
• document the new mode and update generated CLI help
• cover unfiltered, model, search, limit-independent, and tool-filtered counts

Closes #1071

Validation

• uv run pytest tests/test_llm_logs.py -q (83 passed)
• uv run black --check .
• uv run ruff check .
• targeted Cog check for docs/help.md and docs/logging.md

The full suite reached 707 passing tests; its embedding migration failures reproduce unchanged on clean upstream/main with the locally resolved sqlite-utils version (cannot commit - no transaction is active).

[lntutor]

Rebased onto current main and resolved the log-query conflict by adapting --count to the new unified merged_log_rows() path, so it counts both current and legacy log storage while --limit continues to affect display only. Validation: all 87 tests in tests/test_llm_logs.py pass, and Ruff reports no issues for the changed Python files.

[lntutor]

Current head e3affe93742762b4c962a9bef87f87d314363e0d is mergeable, the Read the Docs preview is green, and there are no unresolved review threads on the current PR state. The July 30, 2026 rebase/verification note on this PR still matches the current head. This looks ready for maintainer review when convenient.