CLI club-context: current_club + ccas use <slug>

[Sootopolis]

What

A single active current club so slug-requiring subcommands run with no slug argument — like git switch / kubectl config use-context.

• New config key current_club = "<slug>" in ~/.config/ccas/config.conf.
• ccas use <slug> sets it (writes the config file).
• Subcommands that require a club slug (recruit, stats, membership, history, blacklist *) fall back to current_club when no slug is given; an explicit slug always wins.
• default_clubs (shipped in CLI config file resolution #43) stays the completion-suggestion list; current_club is the single dispatch target.

Why

#43 shipped default_clubs but deliberately did not build the issue's "no slug → use default_clubs, 2+ → error" fallback: a list is only a usable fallback when it holds exactly one, and erroring on every slug-less call when 2+ are set is poor UX. A single current_club is the right model and supersedes the list-as-fallback.

Notes / open questions

• Writing config from the CLI (ccas use) — needs a HOCON writer or a minimal config rewrite; today CliConfig is read-only.
• Where ccas use validates the slug: local DB, GET /api/clubs, or no validation.
• The ServerCommand vs local-command split (from feat(cli): config file resolution + command split #85) makes use a clean new local command.

Relates to #40 (CLI v0). Follow-up from #43 (merged in #85).

[Sootopolis]

Shipped in #123 (squash-merged to main as d18e0f9).

Delivered via a redesign rather than the issue's literal "optional positional slug fallback": positional club slugs are dropped CLI-wide in favour of --club <slug> (comma-separated on membership/history) + --all, falling back to current_club set by ccas use-club <slug> (top-level, local config write). Reason: an optional positional is ambiguous for blacklist add/remove. --all+--club together is rejected. Also renamed the managed-clubs group clubs → club (singular).

Standalone ZIOAppDefault apps still hand-roll argv parsing — out of scope, tracked in #70.

[Sootopolis]

Follow-up fix shipped in #124 (b40352f): club add/remove were left requiring --club (the redesign over-applied to the set-management commands) so ccas club add <slug> dropped the slug — reverted to a required positional <slug>. Also fixed a completion-leaf rename miss (clubs→club) from #123 that had broken club subcommand completion; added a group/leaf drift guard.