Fix all shellcheck warnings for nix build gate

- SC2015: Wrap A && B || C patterns in brace groups for directive scope
- SC2064: Suppress for intentional early trap expansion (local vars)
- SC2164: Add || exit to cd commands in subshells
- SC2001: Suppress for sed URL injection (clearer than parameter expansion)
- SC1083: Handle {tree} git syntax (quote or suppress)
- SC1091: Suppress for runtime-resolved source paths
- SC2034: Remove unused exit codes (E_OK, E_CONFIG, E_AUTH)
- SC2116: Eliminate useless echo in mono_auth_url construction

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/core.sh

@@ -7,10 +7,7 @@ set -euo pipefail
 
 # ─── Exit Codes ─────────────────────────────────────────────────────
 
-readonly E_OK=0
 readonly E_GENERAL=1
-readonly E_CONFIG=10
-readonly E_AUTH=11
 
 # ─── Logging ────────────────────────────────────────────────────────
 # All log output goes to stderr. Sync functions use stdout for return values.