fix(commands): make /sessions CLI-only and functional

[rylena]

Summary

• mark /sessions as CLI-only so gateway platforms no longer advertise or silently swallow it
• wire the classic CLI slash dispatcher to show recent sessions inline when /sessions is used
• add regression tests for command registration and CLI slash handling

Verification

• scripts/run_tests.sh tests/hermes_cli/test_commands.py tests/cli/test_cli_init.py
• independent delegated code review: passed

Notes

• Existing authored PR fix(cli): report passwordless sudo in status #18315 was inspected first; its current red checks reproduce on upstream/main and were not actionable in this run.
• Codex was attempted first, but it failed with the known sandbox / CA-bundle permission errors and a usage-limit stop, so Hermes handled the fallback implementation.

Closes #23533

[alt-glitch]

Overlaps with #23577 which also marks /sessions as cli_only. This PR additionally wires the handler. Related issue: #23533.

[rylena]

Closing this as overlapping with #23577, which already carries the CLI-only /sessions change for #23533. I am not refreshing or pushing this duplicate branch further.