Client/host command classification with role-aware CLI display

[sammy-vastai]

Summary

• Classifies every CLI command as host-only or not (is_host_only_command() in vastai/cli/parser.py), replacing the 26 hand-written [Host] help-text prefixes with an auto-rendered one derived from the classification.
• Adds a stored client/host role (~/.config/vastai/vast_role) that filters --help, tab completion, and error hints — display-only, never gates execution; the server remains the real permission enforcement layer.
• Client is the default. Host-only commands only show once the role is explicitly resolved to host (vastai set role host, or auto-detection in the follow-up PR). An unset or unrecognized role is treated as client, not "show everything" — this is what actually makes fresh installs show the filtered view without requiring the auto-detection in Auto-detect the client/host role from an existing API key #463.
• vastai set role host|client to manually set/override the role.
• A client-role user hitting a machine_read-style 401 on a host command gets a "did you mean" hint (e.g. show machines → show instances).

Auto-detecting the role from an existing API key (rather than requiring set role to be run manually) is split into a follow-up stacked PR: #463.

Test plan

• poetry run pytest tests/cli — 397 passed
• Manually verified: a completely fresh install (no role file, no key) shows the filtered client view by default; set role host/set role client toggle correctly; the 401 hint fires even with an unset role
• Confirmed pre-existing failures in test_tar_utils.py/test_search.py are unrelated (present on master too)

CLN-3582

[sammy-vastai]

Superseded by #484, which combines this and #463 into a single branch (rebased onto current master, conflicts resolved, tests passing).