feat(cli): config file resolution + command split

[Sootopolis]

Closes #43.

What

The ccas CLI now reads optional settings from ${XDG_CONFIG_HOME:-~/.config}/ccas/config.conf:

api_url       = "http://127.0.0.1:8080"
default_clubs = ["team-alpha", "team-beta"]
log_dir       = "~/.local/state/ccas/logs"   # reserved; parsed, not yet used

• Server URL resolves --server <url> flag → config api_url → built-in http://127.0.0.1:8080.
• default_clubs seeds the completion clubs.txt on a fresh install (mtime stamped to epoch so the next server command still refreshes authoritative slugs).
• Missing file → built-in defaults (no crash). Malformed file → error: invalid config file <path>: … (exit 2).

Decisions (diverge from the issue, with sign-off)

• HOCON config.conf, not TOML — reuses typesafe-config already in the tree via zio-config TypesafeConfigProvider.fromHoconFileZIO; no new dependency. (.toml → .conf is the only issue-text deviation.)
• Scope kept minimal — the issue's default_clubs-as-slug-fallback (a list; 2+ → error every call) was deliberately not built; a follow-up issue will design club-context (current_club + ccas use <slug>) which supersedes it.

Refactor folded in

Split CliCommand into ServerCommand (HTTP-client subcommands that carry the resolved server URL) vs local commands (Serve — now a case object with no --server; Completion). Removes the Completion.server = "" sentinel and the dead Serve/Completion branches in Dispatcher; Dispatcher.dispatch/runCommand now take ServerCommand so cmd.server is always real. Regenerated completions/ccas.bash.

Test plan

• sbt test → 797 passed (new TestCliConfig; resolution-order cases in TestCliParser; TestCcasCompletion byte-equality after regen).
• Staged-binary manual checks: missing config → defaults; config api_url used; --server overrides; malformed → clean exit 2; default_clubs seeds clubs.txt; serve --help no longer lists --server.

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