[V2] setup.sh deprecation — move to safer install methods

[ottovlotto]

Per spec v0.31 V2 #90. Deprecate the curl ... | bash install pattern in favour of safer install methods (package manager, signed installer, etc.). Spec flags as may-not-land pre-Summit; install-page warning is the realistic interim. Owner: Charles.

[ottovlotto]

Per spec v0.48 (#90 — setup.sh deprecation): the interim community-code disclaimer landed (src/commands/mod/communityNotice.ts). Full setup.sh replacement is deferred post-Summit (secure replacement non-trivial). Closing the deprecation tracker as post-Summit; the interim mitigation is shipped.