fix(sessions): open read-only commands without writer

[Ahmett101]

Summary
hermes sessions stats and other display-only session subcommands opened a writable SessionDB even though they only read the store. On a live WAL database that transient writer can participate in close-time WAL cleanup and reproduce the reported deleted-sidecar generation. This change routes non-mutating session commands through SessionDB(read_only=True) while leaving mutating commands on the writable open path.

Changes
hermes_cli/sessions_cmd.py: adds a read-only handler allowlist for list, pinned, and stats, and passes read_only=True when dispatching those commands.
tests/hermes_cli/test_sessions_error_exit_codes.py: adds regression coverage that stats opens read-only and a mutating command still opens a writer.

How to Test
python -m pytest tests/hermes_cli/test_sessions_error_exit_codes.py -q

✅ 5/5 passed

ruff check hermes_cli/sessions_cmd.py tests/hermes_cli/test_sessions_error_exit_codes.py

✅ passed

python scripts/check-windows-footguns.py hermes_cli/sessions_cmd.py tests/hermes_cli/test_sessions_error_exit_codes.py

✅ passed

Notes
scripts/run_tests.sh tests/hermes_cli/test_sessions_error_exit_codes.py -q was attempted in the Termux sandbox and failed before collection with PermissionError(13, 'Permission denied'); direct pytest passed.
ruff format --check was not applied because these legacy files would be broadly reformatted unrelated to this fix.

Checklist

• Tests pass — 5/5 targeted direct pytest
• Follows Conventional Commits
• Changes scoped to this fix only
• Cross-platform impact assessed (Linux / macOS / WSL2 / Windows / Termux)
• profile-safe paths used — no hardcoded ~/.hermes
• .env not used for non-credential settings (behavioral settings → config.yaml)

Risk & Impact
Low. The change only selects read-only SessionDB opens for display-only session commands and keeps mutating commands writable.
Type: Bug fix
Closes: #109687

[kshitijk4poor]

@Ahmett101 you were first on the hermes sessions read-only half of #110173; #111627 (salvage of #110186) covers it and credits you as co-author. Closing this one in favour of #111627.

#111627 is armed to auto-merge (rebase) once its CI is green.