feat: allow staff to assign roles on behalf of users

[majestic-owl448]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

Closes #943

Adds /management assign-role, which toggles a role for a target member the same way the self-serve role buttons do. Moderators can now sort out campers who cannot work the onboarding UI.

The role option is autocompleted, and the list is computed live rather than configured: every role below the CamperChan's own highest role, minus the everyone role and minus roles owned by an integration, such as the booster role. Nothing to maintain when roles are added, moved, or removed. The same check runs again when the command executes, since a client can submit any string to an autocompleted option.

Autocomplete had no plumbing in this repository, so the first commit adds it: an optional autocomplete handler on the command and subcommand interfaces, plus a branch in handleInteractionCreate that dispatches to it. Every path there responds, because Discord accepts nothing but a choice list for these interactions and the camper's client hangs until timeout otherwise. The management command routes autocomplete through the same subcommand map as run, and runs the permission validator first, so non moderators get an empty list.

Role changes are posted to the mod webhook, in the same plain content style as the voice reports:

<@moderator> has added the `AI` role to <@camper>.

Discord's audit log credits the CamperChan for the change, so without this there is no record of which moderator made it. The role is named rather than mentioned, so the log cannot ping everyone holding a mentionable role.

No configuration or permission changes are needed to deploy this. Commands are registered on boot, and the Manage Roles permission and the GuildMembers intent are both already in use.

[naomi-freecodecamp]

blocking while discussing on discord